Top Lang Disorders Vol. 25, No. 1, pp. 65-83 © 2005 Uppincott Williams & Wilkins, Inc. Deep-Level Comprehension of Science Texts The Role of the Reader and the Text Rachel M. Best, PhD; Michael Rowe, MS; Yasuhiro Ozuru, PhD; Danielle S. McNamara, PhD Many students from elementary school through college encounter difficulty understanding their science textbooks, regardless of whether they have language disorders. This article discusses some of the particular difficulties associated with science text comprehension and possible remedies for facilitating and enhancing comprehension of challenging expository text materials. Specifically, we focus on the difficulties associated with generating inferences needed to comprehend science texts. The successful generation of inferences is affected by factors such as students' prior knowledge and reading strategies, and the manner in which science texts are written. Many students lack the necessary prior knowledge and reading strategies to generate inferences and thus comprehend science texts only poorly. Further, science texts are typically "low<ohesion" texts, which means that they require readers to generate many inferences andfillin conceptual gaps. Remedies for overcoming comprehension difficulties include matching texts to students' knowledge level and providing explicit instruction aimed at teaching students to use reading comprehension strategies for comprehension monitoring, paraphrasing, and elaborations. The computer-supported tool iSTART Onteractive Strategy Training for Active Reading and Thinking) is introduced as a technological support to assist students and teachers in the teaching/learning enterprise. Key words: comprehension, prior knowledge, reading strategies, science texts, text cohesion A DREADED challenge, even for many stuJLJLdents with typical development, let alone for those with language disorders, is reading the science textbook. Even if students can decode, read, and understand words on the page, the challenge remains to put the words together and make sense of them. For some students, the words remain a string of words, rather than a coherent, comprehensible, and learnable message. Many teachers recognize the problems that students face, as w^ell as the possibility that students may not actually read, let alone comprehend their textbook assignments. This problem is particularly pronounced in difficult subject areas such as science (Bowen, 1999; Snow, 2002). From the University of Memphis, Memphis, Tenn. Comprehension difficulties arise for a numWe express our thanks to Nickola Nelson and three ber of reasons, ranging from poor wordanonymous reviewers regarding their helpful comments on an earlier draft of this article. This material decoding abilities to the inability to use efis based on work supported by grants from the Instifective reading strategies. In this article, we tute of Education Sciences (IES R3056020018-02) and the National Science Foundation (NSF: IERl 0241144) first discuss the cognitive processes involved awarded to the fourth author, Danielle S. McNamara. in the successful comprehension of science Any opinions, findings, and conclusions or recommentexts, drawing on a theoretical framework on dations expressed in this material are those of the authors and do not necessarily reflect the views of IES or the basis of Kintsch's (1998) construction inNSE tegration model (CI model) to explain the role Corresponding author: Rachei M. Best, PhD, or Danielle of inference making in deep-level comprehenS. McNamara, PhD, Department of Psychology, 202 sion. Within the framework of the CI model, Psychology Bldg, The University of Memphis, Memphis, TN 38152 (e-mail: rbest®mail.psyc.memphis.edu we then discuss comprehension problems commonly observed in child and adolescent or d. [email protected]. memphis. edu). 65 66 TOPICS IN LANGUAGE DISORDERS/JANUARY-MARCH populations when reading science texts, including problems resulting from one textrelated factor (text cohesion) and two readerrelated factors (domain-relevant knowledge and reading strategies). Finally, we propose text-based and reader-based remediation techniques to facilitate and enhance science text comprehension and thus overcome comprehension problems associated with reading difficult texts. DNFERENCING AS THE KEY TO DEEP COMPREHENSION OF SCIENCE TEXTS Reading comprehension can be defined as "the ability to obtain meaning from w^ritten text for some purpose" (Vellutino, 2003, p. 51). To comprehend successfully, the reader must identify a series of letters as a word, access the meaning of the word from the lexicon or mental dictionary (Perfetti, 1985), and integrate individual word meanings into a coherent sentence-level representation (Vellutino, Scanlon, Small, & Tanzman, 1994). Deep comprehension, however, requires more than the mere interpretation of individual sentences; the reader must also be able to integrate individual sentence meanings into a coherent text-level representation (Kintsch, 1988, 1998). In other words, to achieve deep comprehension, the reader must construct a global meaning that integrates multiple sentences. The primary focus in this article is on inference generation, a process that leads to text-based and knowledgebased connections to, and across, sentences. The process of creating connections while reading by generating inferences underlies the successful and deep-level comprehension of science texts. Making inferences is a critical feature of understanding the overall meaning of texts (Kintsch, 1988) because inferences combine the individual sentence meanings distributed across the text into a coherent structure (Gernsbacher, 1997). In other words, the meaning of a text often remains fragmented and disconnected without inferences because texts normally do not (or cannot) state all 2005 the information relevant to the situations or events. Therefore, to successfully comprehend a text, the reader must generate inferences to fill in "missing" information and build a coherent mental model that incorporates all the information in the text (Zwaan & Singer, 2003). For example, consider the following sentence pair: 1. Plants lack a nervous system. 2. They carmot make quick responses to stimuli. The first inference that must be made after the first sentence is that the pronominal referent for "they" is "plants." Comprehension also depends on inferring that "the ability to make quick responses" is somehow related to the "nervous system."Building this inferential connection requires the use of logic, syntactic knowledge, and the ability to access knowledge from semantic memory, or to recall relevant information cited in earlier parts of the text. This form of inference is called a "backward causal inference" because it involves attributing the cause of a phenomenon or event described in a given sentence to a thing or event described in a previous section of the text. In the current example, successful, deeplevel understanding of this passage requires the inference that the nervous system is responsible for quick responses. It illustrates the point that successful comprehension of texts requires more than word-decoding, vocabulary, and syntactic skills. Deep-level comprehension also requires the ability to make inferential links across individual sentences in order to construct a global picture of underlying concepts. According to Kintsch's (1988, 1998) CI model, to accomplish this, readers must first use their language-related know^ledge (e.g., morphology, syntax, semantics) to construct a representation on the basis of the information explicitly stated in the text. This initial level of representation is often far removed from a coherent model regarding the situations or events that the text aims to describe; rather the model reflects the information that is explicitly stated in the text. Thus, as illustrated with the plant example, the meaning of a text may remain fragmented and disconnected. Deep-Level Comprehension of Science Texts To go beyond a purely text-level comprehension, the CI model then posits that readers must draw on additional information, such as prior domain knowledge, or information cited in previous parts ofthe text, and integrate this information into a more complete mental representation of the events that the text aims to describe. In this sense, the representation that readers must construct to comprehend a text's deeper meanings does not necessarily correspond w^ith the representation that they can construct directly from information provided explicitly in the text. The related implication of the CI model is that the quality of the text determines the amount and type of inferences that the reader needs to generate in order to comprehend the events or situations in a coherent manner. TEXT COHESION AND INFERENCING The CI model suggests that comprehension can be improved when text cohesion is improved, thus reducing the need to make inferences. This improvement would be particularly true for readers who are unable to generate inferences from difficult texts. Text cohesion refers to properties of the text that determine the degree to which readers need to generate inferences to construct a coherent mental representation from the information explicitly stated in the text. Texts are considered to be low cohesion w^hen constructing a coherent representation from the text requires many inferences. Texts are considered high cohesion when elements within the text provide more explicit clues to relations within and across sentences. Cohesion is important at both the global and local levels of text. The term global cohesion refers to the overall cohesion of the text. Signals for global cohesion include introductory paragraphs, headers, summary paragraphs, and the semantic overlap between paragraphs. Local cohesion concerns the relations between adjacent sentences. Signals for local cohesion include semantic overlap between the sentences, connectives that explicitly describe the relation between sentences. 67 and explanations for difficulty terms or concepts. Texts are considered to be locally cohesive to the extent that the relationship between adjacent sentences (or clauses) is made explicit through linguistic cues. For example, argument overlap can be increased by repeating a referent in a sentence and using fewer pronominal referents or synonyms. Below are examples of sentence pairs that do and do not repeat the target referent "sexual reproduction": 1. We tend to take the existence of sexual reproduction for granted. From an evolutionary standpoint, this is a serious puzzle. 2. We tend to take the existence of sexual reproduction for granted. However, from an evolutionary standpoint, sexual reproduction is a serious puzzle. In this example, the second sentence pair is presumed easier to process because the author has explicitly stated that sexual reproduction is "the puzzle." Also, by adding the connective, however (other examples include therefore, because, etc.), the author has provided more explicit cues for readers on the nature ofthe link between the sentences, generating the expectation among readers that the nature of the puzzle is about to be explained. A second aspect of cohesion is termed explanatory cohesion. This aspect concerns the degree to w^hich the background information necessary to make connections among relevant sentences is provided explicitly by the text. Specifically, the presence of domainspecific background information necessary for the coherent linking of sentences has a large impact on explanatory cohesion. Returning to the previous example about plants, readers would be more likely to recognize the relationship between the two sentences, and to construct a representation that integrated their meanings if the sentences "Plants lack a nervous system" and "They cannot make quick response to stimuli" were preceded by the explanatory sentence "The nervous system is critical for animal's ability to produce quick response to stimuli." Adding explanatory information so that readers do not have to rely solely on background knowledge 68 TOPICS IN LANGUAGE DISORDERS/JANUARY-MARCH helps the reader construct inferences which, in turn, facilitate deeper comprehension. A common problem of many science textbooks, however, is that authors often leave out information they assume to be prior knowledge for target readers. Textbook analyses have revealed that such omissions even occur for information that is critical to the construction of situations or events described by the text (Beck, McKeown, Sinatra, & Loxterman, 1991). Studies also have shown that improving text quality by increasing local and/or global cohesion facilitates comprehension as assessed with a variety of comprehension measures, including recall, open-ended questions, multiple-choice questions, and card-sorting tasks. For example, the positive influence of increasing text cohesion on comprehension is observed with both narrative texts (e.g.. Beck, McKeown, Omanson, & Pople, 1984) and expository texts (e.g.. Beck et al., 1991; Britton & Gulgoz, 1991; Britton, Gulgoz, & Glynn, 1993; Linderholm et al., 2000), especially when readers do not have sufficient domain knowledge to generate inferences necessary for comprehension (McNamara, 2001; McNamara, Kintsch, Songer, & Kintsch, 1996; McNamara & Kitsch, 1996). READER ABBLITY AND INFERENCING Revising a text to increase cohesion can reduce the need for readers to make inferences about relationships among ideas in texts, but there w^ill always be a need for students to generate inferences, if they are to build a mental model of the global concepts conveyed by the text. Several theoretical accounts have been posited for why some readers are able to generate inferences better than others (see McNamara & O'Reilly, 2004, for a review). Some researchers have proposed that readers' ability to generate inferences is a function of their working memory capacity Gust & Carpenter, 1992; Rosen & Engle, 1997; cf. McNamara & Scott, 2001). According to these theories, individuals who can hold and process more information in working memory 2005 are better able to make inferences, because they can hold and process more of the text at the same time (Engle & Marshall, 1983). In this article, we focus on two other important factors that influence readers' abilities to generate inferences. One factor concerns readers' preexisting domain-specific knowledge; the other concerns their degree of competency in using reading strategies. We emphasize roles of domain knowledge and reading strategies because, unlike working memory capacity, they are subject to intervention. Domain-specific knowledge refers to the degree to which readers possess knowledge that is specifically related to the text content. Reading strategies refer to a general set of abilities that facilitate active processing of a text's content, and are considered to be closely related to metacognitive abilities, such as knowledge of cognition and the ability to monitor and regulate ongoing cognitive processes (Hacker, 1998). Domainspecific knowledge and the ability to use reading strategies are not completely independent. Competent readers are able to integrate them and use them in concert. On the other hand, our research suggests that the two factors are distinguishable because they tend to be related to different aspects of the reading comprehension process. Whereas domain-specific knowledge is closely related to how easily one can comprehend material at a given level by accessing relevant knowledge, the ability to use reading strategies is more closely related to the effort and use of active processing techniques, such as elaborative and bridging inferences (Best, Ozuru, & McNamara, 2004; Ozuru, Best, & McNamara, 2004). Contributions of domain-specific knowledge to inferencing With respect to the benefits of domainspecific knowledge, readers with rich and organized topic-relevant knowledge structures (i.e., schema) are at an advantage in making inferences because they can gain quick access to relevant knowledge structures with minimal reliance on explicit text-based input. Deep-Level Comprehension of Science Texts For example, readers are known to generate backward causal inferences (discussed earlier) routinely when the topic of the text is generally familiar, as they are in narrative texts that tend to regard familiar topics such as relationships between individuals and common problems encountered in everyday life (e.g., Graesser, Singer, & Trabasso, 1994). On the other hand, readers are less likely to draw backward causal inferences when reading unfamiliar expository texts (Noordman, Vonk, & Kempff, 1992). These findings suggest that readers typically do not generate backward causal inferences when they have less knowledge about the text topic. Also, a wealth of empirical evidence demonstrates that a reader's background knowledge facilitates and enhances comprehension and learning from expository materials (e.g., Afflerbach, 1986; Chi, Feltovich, & Glaser, 1981; Chiesi, Spilich, & Voss, 1979; Lundeberg, 1987; McNamara & Kintsch, 1996; Means & Voss, 1985; O'Reilly & McNamara, 2002). Having rich domain-specific knowledge seems to be an important factor supporting inference generation, but it is not an essential. Studies have show^n that some aspects of inference generation are observed even w^hen knowledge about a topic is limited. For example, Noordman et al. (1992) conducted two experiments in w^hich they showed that readers are able to draw^ backward causal inferences even when reading expository texts on unfamiliar topics. In a similar vein. Singer, Harkness, and Stewart (1997) found evidence of inference generation for unfamiliar expository materials, that is, as long as the readers were not rushed in their examination of the texts. Together, these studies show that readers sometimes generate inferences even when they possess little domain-specific knowledge, indicating that inference generation depends not only on the reader's knowledge level, but also on other factors. Likely candidates include discourse cues that signal the nature of texts, time constraints on tasks, and readers' perceptions of their reading goals (McKoon & Ratcliff, 1992). 69 Rich domain-specific knowledge also interacts in interesting ways with text-based factors. McNamara and colleagues (e.g., McNamara, 2001; McNamara et al., 1996; McNamara & Kintsch, 1996) investigated the interactions of text cohesion and prior domain-specific knowledge in text comprehension by manipulating text cohesion and measuring the effects on comprehension by low- and high-knowledge readers at the middle-school and college levels. Interestingly, whereas low-knowledge readers demonstrated better comprehension when reading high-cohesion texts, high-knowledge readers experienced better comprehension when reading low-cohesion texts. One interpretation of this finding is that the redundancy between information in high-cohesion texts and background knowledge led highknowledge readers to engage in passive processing, thus keeping their comprehension at a surface level (see also Gilabert, Martinez, & Vidal-Abarca, 2005). According to this explanation, increased text-based cohesion may interfere with high-know^ledge readers' ability to actively process texts by decreasing the need for spontaneously generated inferences. In contrast, low-cohesion texts require readers to generate many inferences. Thus, high-knowledge readers are induced by low-cohesion text to work more actively to integrate information in the text with their prior know^ledge. This active processing allows high-know^ledge readers to achieve deeper levels of comprehension. However, low-knowledge readers do not possess the knowledge necessary to generate the inferences required by low-cohesion text. As a consequence, low-know^ledge readers are disadvantaged by a combination of low text-cohesion and low domain-specific knowledge. There is also evidence that readers with high reading ability (as measured with the Nelson-Denny test; Brown, Fishco, & Hanna, 1993) and high domain knowledge perform equally well when reading either lowcohesion or high-cohesion versions of texts (O'Reilly & McNamara, 2004). They found 70 TOPICS IN LANGUAGE DisoRDERS,(fANUARY-MARCH 2005 that only high-knowledge readers with low scores on the Nelson-Denny Reading Test benefited from reading low cohesion texts. In contrast, high-knowledge readers with higher scores on the Nelson-Denny Reading Test benefited from reading high-cohesion texts. This pattern of results indicates that whereas high-knowledge readers with lower reading abilities tend to process texts actively only when the low-cohesion version of the text demands the active generation of inferences, high-know^ledge readers with good reading abilities tend to actively process texts regardless of the text cohesion level or task demands. This research suggests that how actively a reader processes text by generating inferences may be influenced not only by the reader's domain-specific knowledge but also by an ability to use knowledge actively and strategically when reading science texts. In the next section, we consider the role of reading strategy use in generation of inferences that facilitate deep-level comprehension of science texts. Contributions of reading strategies to inferencing Solely having domain-specific knowledge is often not enough; to be used effectively, readers must deliberately activate their knowledge or it may remain somew^hat "inert" and fail to contribute to deeper understanding. That is, students may have some domain and general knowledge that can be used to generate inferences and thus understand a text at a deeper level, but they may not realize that their knowledge is applicable to a particular reading situation. Indeed, several researchers have found that readers sometimes encounter difficulty invoking preexisting knowledge in novel situations (e.g., Bransford, 1979; Hasher & Zacks, 1979; Nitsch, 1977). This situation is where reading strategy contributions come into play. Competency in reading strategy use provides an additional factor that influences inference generation while reading science texts. The notion that reading strategy use facilitates inference gen- eration (and hence, deep comprehension) is bolstered by several sources of evidence. According to McNamara and Scott (2001), the use of reading strategies, such as the careful monitoring of text contents at the time of reading, is an important component of reading comprehension ability. Good comprehenders (e.g., as assessed using the Nelson-Denny Reading Comprehension Test) are more likely to generate inferences that repair conceptual gaps between clauses, sentences, and paraphrases (Cain & Oakhill, 1999; Long, Oppy, & Seely, 1994; Magliano & Millis, 2003; Magliano, Wiemer-Hastings, Millis, Munoz, & McNamara, 2002; OakhiU, 1982,1984; Oakhill & Yuill, 1996). Furthermore, good comprehenders have more metacognitive knowledge (Baker, 1982; Wong, 1985) and are more likely to use reading strategies to repair gaps in their understanding than poor comprehenders (e.g.. Garner, 1987; Long & Golding, 1993; Oakhill, 1982, 1983). Finally, interventions that promote the active and strategic use of knowledge has been show^n to improve reading comprehension (e.g., Bereiter & Bird, 1985; Bulgren, Deshler, Schumaker, & Lenz, 2000; Chi, de Leeuw, Chiu, & La Vancher, 1994; Cornoldi & Oakhill, 1996; Kucan & Beck, 1997; McNamara, 2004, McNamara & Scott, 1999; O'Reilly, Best, & McNamara, 2004; O'Reilly, Sinclair, & McNamara, 2004a, 2004b). Collectively, these studies indicate that reading strategies are important to successful comprehension, and that readers can be taught strategies to improve their comprehension. CHALLENGES OF COMPREHENDING SCIENCE TEXTS Students of all ages have been found to experience difficulty comprehending and learning from science texts (Brand-Gruwel, Aarnoutse, & Van den Bos, 1998; Nichols, Rupley, & Willson, 1997). The problems with science textbook comprehension, however, are particularly pertinent among students at the elementary-school and middle-school levels, when children are first exposed to them. Deep-Level Comprehension of Science Texts Science texts contain difficult vocabulary and syntax, and also place greater emphasis on inferential thinking and the use of prior knowledge (Allington, 2002). The increased exposure to challenging expository materials, including science textbooks, at a time when domain knowledge is still developing places greater cognitive demands on young readers, and may account for some of the reading comprehension difficulties experienced by children in the third to fifth grades, which is sometimes called the fourth-grade slump (McNamara, Floyd, Best, & Louwerse, 2004; Meichenbaum & Biemiller, 1998; Sweet & Snow, 2003). In this section of the article, we discuss factors that account for why the comprehension of science text is particularly difficult for many students. We focus on three factors of reading comprehension identified earlier in the discussion (text quality, background knowledge, and reading skill). Our goal is to provide a detailed picture of the nature of problems commonly experienced by students in this particular curricular area to assist clinicians in devising possible remedies. KNOWLEDGE DEFICITS AND MISCONCEPTIONS As noted previously, the CI model of text comprehension emphasizes domain-specific knowledge as an important factor driving text comprehension. Further, a number of studies indicate that knowledge is, arguably, the most important factor determining expository text comprehension (e.g., Afflerbach, 1986; Chi, Feltovich, & Glaser, 1981; Chiesi, Spilich, & Voss, 1979; Lundeberg, 1987; Means & Voss, 1985). According to this view, it is inevitable that many students experience significant difficulty comprehending expository texts and, in particular, texts dealing with scientific concepts because their existing knowledge is limited. Science textbooks, along with other expository texts, are used in the classroom for the purpose of providing ne^v information to students. Thus, it is not surprising that most students in the classroom have a little to 71 no background knowledge about the domainspecific expository text content when they first encounter it. Students' knowledge deficits may take, at least, two different forms. First, many students may lack the knowledge of specific concepts outlined by the text (e.g., osmosis, gravity, etc.). As our earlier plant example indicated, w^hen students do not have a sufficient understanding of a particular concept, they often have a problem generating inferences to link concepts within or across sentences. As a result, their understanding of the text remains fragmented and isolated, causing a failure to form a coherent mental representation of the overall text content. Second, students' knowledge deficits may take the form of preexisting misconceptions based on common knowledge or personal experience, rather than scientific concepts. Understanding scientific phenomena often requires adopting a completely different perspective from that acquired from everyday perceptual experiences. Indeed, Vygotsky (1978) pointed out the problem of integrating spontaneously developed knowledge with "scientific" or "theoretical" concepts, which are abstract in nature and cannot be acquired from direct perceptual experience. For example, children's direct perceptual experience of the movement of the sun in the sky can create the misunderstanding that the sun moves across the sky. This perceptual experience can make it difficult for children to understand that the sun moves as a function of the earth rotating and revolving around the sun, and not the sun's movement around the earth. Children with misconceptions about the movement of the sun may fail to comprehend a text about the solar system accurately because they cannot reconcile information stated in the text with preexisting background knowledge gained from everyday perceptual experiences. According to the CI model, the utilization of preexisting misconceptions (e.g., incorrect background knowledge) in reading comprehension can give rise to the construction of an inaccurate mental model of the situation (by making inappropriate 72 TOPICS IN LANGLIAGE DISORDERS/JANUARY-MARCH inferences), which contrasts with the scientific model of the situation intended by the authors of the text. Such misconceptions may give rise to what Piaget (1985) described as the "assimilation" of the incoming information to the preexisting knowledge structure, instead of "accommodation" of the student's current knowledge structure to the text. Thus, for students with misconceptions, deep, or accurate, text comprehension requires not only accessing and using prior background knowledge, but also recognizing misconceptions, or contradictions between their prior knowledge and the text content. Perhaps most importantly, successful comprehension requires that readers subsequently repair any erroneous aspects of their mental models (Chi et al., 1994). TEXT COHESION INFLUENCES REVISITED As discussed earlier, several studies have shown text with high cohesion to be particularly beneficial to readers with less knowledge about the text domain (Beck et al., 1991; Britton et al., 1993; Britton & Gulgoz, 1991; McNamara, 2001; McNamara et al., 1996; Vidal-Abarca, Martinez, & Gilabert, 2000). It is important to bear in mind that there is not an optimal level of explicitness or cohesion for all readers. Rather, comprehension success also depends on the knowledge level and reading strategies that the reader brings into the reading situation. As mentioned earlier, high-cohesion text is not necessarily helpful to high-know^ledge readers with a low level of competency in reading strategy use. It is theorized that this reverse cohesion effect occurs because high-knowledge readers are more likely to actively engage with and comprehend texts that contain fewer cohesion cues. However, many, if not most, students lack the necessary domain-specific knowledge to generate inferences when reading their science textbooks, particularly if the books are w^ritten with low cohesion. The point here is that students' knowledge deficits often are compounded by a general lack of cohesion in science textbooks. 2005 Moreover, there is reason to believe that the selection of science texts may be carried out by textbook selection committees without sufficient attention to all of the textual features that influence the understandabUity of the texts. First, texts are often classified and selected on the basis of traditional readability formulas that rely on simple indices such as word frequency, word length, and sentence length (Beck et al., 1991; Britton et al., 1993). According to these formulas, texts that comprise short words and sentences are considered less difficult, and thus more suitable for novice readers. Short words tend to be more frequently used and encountered, and thus are considered to be de facto more familiar (Zipf, 1932). Short sentences place fewer processing demands pertaining to lower level cognitive process (e.g., syntactic processing), but short sentences do not necessarily facilitate deep-level comprehension because they offer fewer cohesion supports for low-knowledge readers. That is, texts characterized by short words and short sentences are likely to lack adequate cohesion because the sentences are not likely to contain connectives. In addition, a focus on readability in terms of short words and sentences is likely to diminish the focus on elements of cohesion such as referential and explanatory cohesion. The lack of cohesion supports built into the text places other demands on the reader, such as the need to make inferences about the nature of the links within and between sentences, which will tend to interfere with higher level processing. In sum, a reliance on readability formulas is likely to result in a lack of attention to other textual properties that have an important bearing on deep-level text comprehension. A second complicating factor is that textbooks are written by experts on the topic. Experts' high-level knowledge of a subject matter can interfere w^ith writing for less knowledgeable readers such as school students (Britton et al., 1993). A large body of evidence indicates that people have difficulty taking a perspective of others who are dissimilar to themselves (for a review, see Nickerson, Deep-Level Comprehension of Science Texts 1999). In other words, vast domain knowledge, which is obvious and natural to domain experts, can lead them to assume an unrealistic amount of prior knowledge on the part of readers (Beck et al., 1991). As a consequence, textbooks often lack sufficient background information for non-domain-expert readers to understand the content, impeding the formation of a coherent situation model in these readers' minds. READING STRATEGY EVELUENCES REVISITED As reported earlier, the use of strategic reading practices, such as the active generation of inferences using backward causal inferencing, plays a critical role in the deep-level comprehension of science texts. However, there are reasons to believe that it is difficult to attain sufficient competency in reading strategy use by the time children begin to read challenging science texts. We discuss two possible reasons why students struggle to acquire and apply reading strategies. First, students, particularly those at the elementary school level, may not have mastered efficient and accurate lower level comprehension skills (e.g., efficient word decoding), which are necessary for the execution of higher level comprehension processes (e.g., strategic reading practices, metacomprehension, backward causal inferences). Slow or inaccurate word recognition skills may affect comprehension by consuming too much working memory capacity, thus constraining resources that could be used for deep comprehension processes involving inference generation (Cain, Oakhill, & Bryant, 2004; Curtis, 1980; Hannon & Daneman, 2001; Perfetti, 1985). Many studies have indicated that children may not have mastered word decoding skills by the time they are introduced to expository materials (Brand- Gruwel, Aarnoutse, & Van den Bos, 1988; Mommers, 1987; Roth, Speece, Cooper, & De La Paz, 1996; Taschow, 1969; Vellutino, 2003) and, thus, are stUl struggling with reading fluency w^hen confronted with texts that introduce challenging new concepts. 73 Second, even if students have acquired sufficient word decoding ability, they still may not have attained sufficient competency in using reading strategies necessary for successful comprehension and learning from science texts. For many students, strategic reading practices needed to support inference generation are unlikely to develop automatically just by virtue of reading narrative texts in classroom reading activities. Instead, knowledge of reading strategies and effective use of these strategies in comprehending challenging expository texts may need to be explicitly taught. A report published by Educational Testing Service (ETS, 2004) on current fourth-grade reading instruction indicated that teachers do provide students with instructions on techniques such as "predicting contents of the material they are reading" and "making generalizations and inferences about reading contents"(p. 24). However, there is little evidence that deep-level reading strategies are explicitly taught as a part of the normal curriculum. It seems that the majority of reading comprehension training administered at school to children focuses on lower level reading skills such as efficient word decoding skills (Mommers, 1987; Nichols et al., 1997; Pressley & Wharton-McDonald, 1997; Roth et al., 1996; Wilson & Rupley, 1997). It is a concern if such instruction occurs in the absence, or at the expense, of the explicit teaching of higher level processes, such as strategic reading practices. Investigations regarding the use of metacognitive strategies, in fact, have indicated that teachers spend little time giving metacognitive and strategy-oriented instruction pertaining to reading comprehension (Baker, 1996; Graesser, Person, & Magliano, 1995; Kurtz, Schneider, Carr, Borkowski, & Rellinger, 1990; Moley et al, 1992). Furthermore, reading problems for children who have learned higher level reading skills such as inference generation strategies in the context of texts with a familiar narrative structure may only become apparent when they are faced with the additional demands of expository texts about unfamiliar topics. The crux 74 TOPICS IN LANGUAGE DISORDERS/JANUARY-MARCH of the problem is that students are less likely to generate inferences when dealing with unfamiliar materials, such as expository science texts (Noordman et al., 1992), even though this is exactly the situation in which they need to generate inferences for understanding and learning new information from the texts. In sum, it is important not to ignore the instruction of comprehension strategies at early stages of reading development. Young readers need explicit instruction in decoding words, but also need explicit instruction in how to strategically comprehend complex information. REMEDIATION STRATEGIES Thus far, we have emphasized the importance of inference generation in the comprehension of science texts and the difficulties that many readers encounter generating inferences when reading science texts. The final goal of this article is to discuss possible remediation strategies to enhance science text comprehension. The discussion focuses on two lines of research currently being pursued in our research laboratory at the University of Memphis. One involves text difficulty, specifically the ways in which we can assess text difficulty and select texts most likely to facilitate comprehension among different kinds of readers. The other involves designing instructional programs to teach readers active reading practices based on metacognition and reading strategies that facilitate the deep-level comprehension of science texts. Text-based modiflcations: Identiiying texts appropriate for the reader As the prior discussion has indicated, text characteristics have an important impact on the readers' deep-level comprehension of science texts, particularly for readers who have less domain knowledge, which, we believe, comprise a majority of school students reading science texts for the purpose of acquiring knowledge. Reviews of existing research indicate that many of the expository texts 2005 used in actual classrooms are too demanding for students who lack sufficient background knowledge because they are "low cohesion" texts that require the reader to generate many inferences, •written by experts who overestimate students' background knowledge. Given this situation, it is critically important for researchers and educational practitioners to find means to identify and select texts that are more appropriate for students, especially for those who lack necessary domain knowledge or who have language disorders that interfere with their ability tofillin the gaps. As Britton et al. (1993) and others (e.g.. Beck et al., 1991) have pointed out, current text classification systems and selection criteria (e.g., reading grade level formulas) appear to be oversimplistic, focusing on surface attributes of the text (e.g., word and sentence length) that affect relatively low levels of processing. One of the reasons for the use of simplistic methods is there has been no alternative method to analyze and classify text difficulty in a systematic manner (Britton et al., 1993). Readability measures do not take into account local cohesion (i.e., argument overlap between adjacent sentences) or explanatory cohesion, which contribute critically to how easily a reader processes a text. Thus, a more adequate assessment of text difficulty would include measures of local and global cohesion. Work being conducted in our lab has led to the development of a computerized tool, called Coh-Metrix (Graesser, McNamara, Louwerse, & Cai, 2004), which is designed to analyze various features of texts, including cohesion. The tool automatically measures text characteristics predictive of text difficulty using the latest techniques available in computer science and computational linguistics. Coh-Metrix provides measures of text cohesion as well as other text difficulty measures (e.g., word frequency, average sentence length, and type token ratio). The cohesion measures focus on the number of connectives, pronouns, degree of argument overlap between sentences, and conceptual overlap between sentences across Deep-Level Comprehension of Science Texts adjacent and distant sentences using Latent Semantic Analysis (Landauer & Dumais, 1997), which was developed by researchers at the University of Colorado. As discussed earlier, the number of connectives contained in a text is an important measure of cohesion. Connectives provide explicit cues for readers when generating inferences. Similarly, argument overlap, which is measured by the degree to which adjacent sentences share content words, is known to be related to local cohesion because having lexical overlap helps readers relate sentences and construct higher text level representation. However, simple word matching may not provide a comprehensive measure of overlap. As described by Landauer and Dumais (1997), Latent Semantic Analysis (LSA) bolsters argument overlap analysis by computing the co-occurrence of two or more Unguistic units (w^ords, phrases, sentence, etc.) with respect to a given reference point within the LSA space. The LSA space is a mathematical, dimensional space for plotting vectors derived from the linguistic units (e.g., sentences) being compared. Landauer and Dumais showed that LSA betw^een two linguistic units correlates w^ith the conceptual similarity among linguistic units, not just the identical words each unit possesses. The LSA is therefore useful for measuring the approximate cohesion between two sentences in terms of the overlap of conceptually related w^ords, not just the words themselves. Such a conceptual overlap between words would not be captured in a simple analysis of argument overlap (i.e., word matching). Ultimately, the Coh-Metrix tool is designed to be used by researchers and educational practitioners to measure aspects of texts that contribute to reading comprehension difficulties. Although the tool does not yet include established criteria for distinguishing low-cohesion texts from high-cohesion texts, it does offer information about differences in levels of cohesion measures. Researchers and educators can then use these as a guideline for selecting texts that are more or less cohesive and thus more appropriate for read- 75 ers with more or less domain knowledge. A Coh-Metrix analysis of texts from published studies of text cohesion (McNamara, Ozuru, Louwerse, & Graesser, 2005) indicates that the tool reliably captures critical aspects of text manipulations (argument overlap, increase of explanatory cohesion) such as those used in experimental research of text revision (e.g.. Beck et al., 1991; McNamara et al., 1996; Voss & Silfies, 1996). Overall, we believe that research investigating text features will provide a more systematic and reliable method of measuring and classifying text difficulty. Furthermore, new^ measures of text difficulty should offer an important alternative to the simplistic measures currently used (e.g., reading grade level formulas). Consequently, tools such as CohMetrix are anticipated to positively contribute to the process of assigning more appropriate texts to students who may lack sufficient background knowledge. Reader-focused interventions: Teaching reading strategies If problems that students encounter understanding their science textbooks stem largely from deficits in, or erroneous, background knowledge, it is important to find remedies that compensate for "poor knowledge" by teaching them comprehension strategies. Even if a tool such as Coh-Metrix (Graesser et al., 2004) becomes capable of providing information that helps improve text cohesion, it is impractical or even impossible to provide all the cohesion cues and background information necessary for low-knowledge readers. In addition, such an overly cohesive text might interfere with the active processing that is necessary for deep-level comprehension by higher ability readers (e.g., McNamara et al., 1996). Thus, reading comprehension difficulties relating to the comprehension of science texts need to be approached not only from the perspective of the text, but also from the perspective of the reader. As we have noted throughout the article, it is the nature of education that readers of science texts read them for the purpose of 76 TOPICS IN LANGUAGE DisoRDERS/jANtJARY-MARCH 2005 learning new information. Thus, most readers approach the task of reading science texts with low levels of topic-relevant knowledge. The circularity of this phenomenon is such that readers cannot comprehend the text contents at a deep level without learning new concepts or information from the text. This contrasts w^ith the reading process of narrative texts, where comprehension is supported by sufficient knowledge about the content so that inferences necessary for deep-level comprehension may be generated relatively automatically. The comprehension processes necessary for science texts, therefore, require greater effort because the information relevant to understanding a given sentence, or relations between sentences, is not often readily accessible in long-term memory. Readers often need to search for relevant information through various forms of linking (e.g., association, analogy). Effortful generation of inferences is psychologically different from the inferences that readers generate to understand familiar narrative materials (see Kintsch, 1993). For this reason, not only readers, but also instructors, need to be aware of the nature of the difficulties associated with reading comprehension and learning from science texts. They also need to actively deal with these problems by (1) explicitly discussing with students the nature of comprehension problems pertaining to science texts and (2) teaching and using techniques that help students with limited prior topic knowledge deal effectively and strategically with the challenges of science text comprehension. Numerous reading strategy interventions have been developed and tested. There is converging evidence to show that the provision of explicit reading strategy training. In which readers are taught and trained to actively process a text using specific reading strategies, is effective for facilitating deeper level comprehension of texts. These intervention programs vary from those focusing on the training of lower level skills (Adams, 1990), motivational aspects of reading (Guthrie, 2003), and higher level strategies provided to low-achieving adolescents with comprehension difficulties and language disorders (Bulgren et al., 2000; Deshler, & Denton, 1984; Fisher, Schumaker, & Deshler, 2002). Our aim here is not to provide an exhaustive list of interventions, but to show the ways in which reading strategies can help students overcome knowledge deficits. We focus predominantly on strategy training developed in our laboratory at the University of Memphis for typically developing students from the middle-school level and beyond (i.e., students most frequently exposed to science texts). Generally, our efforts have concentrated on teaching reading strategies to help typical students who lack topic-relevant knowledge for comprehending science texts. In the remainder of this section, we first describe four specific reading strategies that are known to contribute to active processing of science texts, and then describe Self-Explanation Reading Training, a reading strategy training intervention program developed in our lab that provides instruction and practice on the use of these specific strategies. Comprehension monitoring Students' ability to monitor comprehension critically and accurately is the foundation of strategic and active reading (Hacker, 1998). That is, readers need to be aware of not only whether they are having comprehension problems, but also the nature of the problem they encounter (e.g., word meaning, syntax, or relations between sentences). However, there is evidence that most readers are rather poor at monitoring their own comprehension (e.g., Glenberg, Wilkinson, & Epstein, 1982). Thus, providing training to improve comprehension monitoring is important to promote the active and strategic processing of expository texts. Specific methods of teaching comprehension monitoring include selfquestioning and checking texts for content consistency. Research conducted with elementary school students has indicated that "poor comprehenders" benefit from comprehension monitoring training in w^hich they are taught to self-question what they understand Deep-Level Comprehension of Science Texts about the material and to check the text for content consistency (Baker, 1985). Paraphrasing Paraphrasing is an important technique for facilitating the active processing of texts (Rosenshine & Meisler, 1994). Paraphrasing requires transforming the surface characteristics of the sentence by replacing the content words or syntactic structure of the sentence with equivalent forms, hence forcing readers to process the information actively by accessing related but different lexical items. Paraphrasing also externalizes one's understating of the information in the text, which, in turn, helps readers monitor comprehension more closely. Difficulty when paraphrasing is a clear sign of a comprehension problem. Elaboration Generating elaborative inferences based on the reader's personal experience or commonsense knowledge relating to information described by the text is assumed to be useful for facilitating deep-level comprehension (McNamara, 2004). For one thing, elaborative inferences help readers overcome gaps in domain-specific knowledge. As our discussion has indicated, understanding texts at a deep level requires the reader to generate inferences tofillin gaps present in text-based information, such that information distributed across text can be encoded as coherent and in an integrated manner. Also, the use of prior knowledge is important for helping the reader retain novel information learned from text in memory; new^ information cannot be learned and retained without being integrated with prior knowledge (Pressley et al., 1992). Thus, when readers do not have sufficient domain knowledge, they need to forcefully integrate the new information in the text with existing knowledge by forming a link between the new information and indirectly related general knowledge or personal experience. As discussed earlier, however, relying on general knowledge and/or personal experience alone may lead to the formation of an inaccurate situation model (e.g., Vygotsky, 1978). This 11 problem can be minimized if readers combine elaborations based on general knowledge and personal experience with inferences using information cited in the prior sections of the text. Bridging The process of generating inferences using information stated in previous sections of the text plays an integral role in helping readers build a global representation ofthe text. More cohesive texts provide a stifficient amount of background information for readers to build a global understanding of the overall text by continuously adding and integrating newly introduced information with previously cited information (Clark & Haviland, 1977; Gernsbacher & Hargreaves, 1988). Although many school texts may not contain sufficient background information and the cohesion cues that help readers link information presented in different sentences in an appropriate manner (e.g.. Beck et al., 1991), students should be taught to maximize the use of information provided within a text to understand the meaning of individual sentences as well as the overall meaning of the text. When struggling to disambiguate and/or narrow down the meaning of a given sentence, readers need to learn that information stated in the previous section can be a powerful and reliable source of information that can aid their comprehension. Self-Explanation Reading Training (SERT) In summary, we postulate that the combination of the above-mentioned reading strategies constitute a powerful combined tool for helping readers comprehend and learn from expository texts. The essence ofthe approach is to teach readers to compensate for both knowledge and text deficits by learning to draw deeper inferences based on the active, constructive processing of connections that can be found in texts even when the cues are not immediately obvious. In our lab we have developed a reading strategy training program for promoting the 78 TOPICS IN LANGUAGE DISORDERS/JANUARY-MARCH active and strategic processing of science texts by combining the above specific reading strategies through a method called selfexplanation. Self-explanation refers to the process of explaining text contents to oneself while reading (Chi, Bassok, Lewis, Reimann, & Glazer, 1989). There are a number of benefits to using the self-explanation method, including facilitating inference generation and repairing erroneous mental models by increasing the extent to which the reader monitors the ongoing comprehension process (Chi, de Leeuw, Chiu, & LaVancher, 1994). Our program, called Self-Explanation Reading Training (SERT; McNamara, 2004; McNamara & Scott, 1999) uses the selfexplanation technique to teach effective reading comprehension strategies (monitoring comprehension, paraphrasing, elaborative inferences, and generating bridging inferences). SERT is divided into three components: (1) Introduction, in which students are taught the reading comprehension strategies; (2) Demonstration, in which the strategies are demonstrated to students; and (3) Practice, in which students practice self-explaining science texts using the reading strategies. In this way, our program aims to promote both explicit understanding of reading strategies (declarative knowledge) and the skills of applying the strategies w^hUe reading challenging science texts (procedural knowledge). Thus far, research conducted with students in middle-school, high-school, and college levels has indicated that SERT increases selfexplanation quality and subsequent comprehension of science texts. Eor example, a study conducted with 38 middle-school students (O'Reilly & McNamara, 2004) demonstrated that students trained with SERT performed better on a comprehension assessment than untrained students. In a study conducted with 42 college undergraduates, McNamara (2004) found that SERT improved the quality of self-explanations (as reflected in students' increased use of elaboration and bridging strategies) and subsequent comprehension of science texts and that low-knowledge students benefited most from SERT training. Likewise, 2005 O'Reilly, Best, and McNamara (2004) found that SERT improved science text comprehension among low-know^ledge high-school readers. In this study, low-knowledge readers trained by SERT performed better on a comprehension task than low-knowledge readers trained using a different form of reading strategy training (Previewing) or low-knowledge readers assigned to a control condition. Current research in our lab has begun to focus more closely on the effects of individual differences in reading strategy intervention, with the aim being to build a studentadaptive reading strategy tutor called Interactive Strategy Training for Active Reading and Thinking (iSTART; McNamara, Levinstein, & Boonthum, 2004). In its prototype, iSTART is an automated, interactive tutor, which currently incorporates SERT but is being expanded to include other reading strategies. The current iSTART system continuously evaluates what students know about the strategies and their ability to use the strategies so as to tailor scaffolding and feedback to the level of the student. Eor example, students struggling to produce elaborations are encouraged to "add more details" to their self-explanations. Evaluations of iSTART indicate that the system improves the students' use of reading strategies and comprehension of science texts at both the middle-school and college level (Magliano et al., 2005; O'Reilly et al., 2004a, 2004b). Our future research will continue to explore the relations between individual differences and the benefits of reading strategy training to develop reading strategy interventions that are increasingly adaptive to each student's needs. Preliminary findings suggest that automated versions of reading strategy trainers are useful not only for individual intervention but also for assisting teachers to implement strategy training in educational settings such as the classroom. One of the factors impeding students' ability to acquire and practice effective reading comprehension techniques stems from the limited explicit instruction regarding strategies for comprehending expository texts. The low level of explicit instruction Deep-Level Comprehension of Science Texts in classroom settings may, in part, be caused by limitations in teachers' knowledge about reading strategies, or a limited allocation of classroom teaching time. Computerized programs may make it easier to implement strategy training in schools because they can reduce the demands on teachers. CONCLUSIONS This article has addressed problems students experience in comprehending their science textbooks by focusing on how attributes of texts (specifically factors involving text cohesion) interface with attributes of readers (specifically world knowledge and discourse knowledge and strategies). Our discussion of remediation strategies has considered the ways in which researchers and educators can help students better comprehend the language of their science textbooks. One method is to match readers to texts, such that lowknowledge readers are given more cohesive texts to help overcome prior knowledge deficits. Text analysis tools (e.g., Coh-Metrix) allow us to grade text difficulty appropriately and distinguish low-cohesion from highcohesion texts. Alternatively, research into 79 text cohesion may be used to develop guidelines for future textbook writing, or to open up the possibility of assigning different textbooks to different individuals. A second method to overcome comprehension difficulties is to teach students reading strategies. Many forms of strategy training are available to help students of various ages and levels overcome problems regarding the deeplevel comprehension of science texts. From our perspective, techniques that help students identify problems in their understanding (e.g., comprehension monitoring) should be combined with instruction aimed at overcoming deficits in prior knowledge (e.g., elaborative inferencing), with the ultimate goal being deep-level comprehension. 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