Blocked: When the Information Is Hidden by the Visualization Kyong Eun Oh School of Library and Information Science, Simmons College, 300 The Fenway, Boston, MA 02115. E-mail: [email protected] Daniel Halpern School of Communications, Pontificia Universidad Catolica de Chile, Alameda 340, Santiago 8331150, Chile. E-mail: [email protected] Marilyn Tremaine, James Chiang, and Deborah Silver Electrical and Computer Engineering, Rutgers, the State University of New Jersey, Piscataway, NJ 08854. E-mail: [email protected]; [email protected]; [email protected] Karen Bemis Institute of Marine and Coastal Sciences, School of Environmental and Biological Sciences, Rutgers, the State University of New Jersey, 79 Dudley Road, New Brunswick, NJ 08901. E-mail: [email protected] This study investigated how people comprehend threedimensional (3D) visualizations and what properties of such visualizations affect comprehension. Participants were asked to draw the face of a 3D visualization after it was cut in half. We videotaped the participants as they drew, erased, verbalized their thoughts, gestured, and moved about a two-dimensional paper presentation of the 3D visualization. The videorecords were analyzed using a grounded theory approach to generate hypotheses related to comprehension difficulties and visualization properties. Our analysis of the results uncovered three properties that made problem solving more difficult for participants. These were: (a) cuts that were at an angle in relation to at least one plane of reference, (b) nonplanar properties of the features contained in the 3D visualizations including curved layers and v-shaped layers, and (c) mixed combinations of layers. In contrast, (a) cutting planes that were perpendicular or parallel to the 3D visualization diagram’s planes of reference, (b) internal features that were flat/planar, and (c) homogeneous layers were easier to comprehend. This research has direct implications for the generation and use of 3D information visualizations in that it suggests design features to include and avoid. Received May 21, 2014; revised November 18, 2014; accepted November 18, 2014 C V March Online 2015 inLibrary Wiley Online Publishedonline online 21 in Wiley © 2015 2015 ASIS&T ASIS&T Published • Library (wileyonlinelibrary.com). DOI: 10.1002/asi.23479 (wileyonlinelibrary.com). DOI: 10.1002/asi.23479 Introduction Three-dimensional visualizations are widely used in information visualization to aid in comprehending large amounts of information. One of the widely used definitions of information visualization is “the use of computersupported interactive visual representations of abstract data to amplify cognition” (Card, Mackinlay, & Shneiderman, 1999, p. 6). Another definition of information visualization by Gershon and Page (2001) is “A process that transforms data, information and knowledge into a form that relies on the human visual system to perceive its embedded information” (p. 32). There have been a number of studies focussing on visualizations on the web (Heo & Hirtle, 2001; Modjeska & Chignell, 2003). In addition, types of available visualization tools and applications are increasing in number. However, many of them are often designed and used without careful examination of their characteristics or of prior empirical research, which suggests that such visualizations cause serious user comprehension problems (Heo & Hirtle, 2001; Keller & Tergan, 2005). Visualizations can help users better understand information; however, they also can hinder users’ interactions with information when not used properly. Thus, it is critical to understand what aspects of visualization enhance or reduce its effectiveness. JOURNAL JOURNAL OF OF THE THE ASSOCIATION ASSOCIATION FOR FOR INFORMATION INFORMATION SCIENCE SCIENCE AND AND TECHNOLOGY, TECHNOLOGY, 67(5):1033–1051, ••(••):••–••, 2015 2016 Among various visualization techniques, 3D visualization, which attempts to provide the visual experience of the real world to users, has become one of the popular and promising visualization techniques. In particular, 3D visualizations can provide rich information about concepts and their relationships and thereby support users’ understanding of the information presented. However, 3D visualizations have routinely been found to be difficult to comprehend solely because of a viewer’s inability to comprehend the visual relationships in the presentation (Fabrikant, Montello, & Mark, 2010; Kali & Orion, 1996; Velez, 2009; Velez, Silver, & Tremaine, 2005). This problem is exacerbated by a viewer’s spatial ability, with low spatial individuals having the most difficulties (Cohen & Hegarty, 2007b; Keehner, Hegarty, Cohen, Khooshabeh, & Montello, 2008; Khooshabeh & Hegarty, 2010; Modjeska & Chignell, 2003; Velez, 2009; Velez et al., 2005). In addition, research shows that spatial ability is related to gender, with females repeatedly scoring lower than males on standard spatial ability tests (Baenninger & Newcombe, 1989; Czerwinski, Tan, & Robertson, 2002; Hyde, 1990; Johnson & Meade, 1987; Kimura, 1999; McGee, 1979; Tan, Czerwinski, & Robertson, 2003; Velez, 2009; Velez et al., 2005). Because modern science and technology rely increasingly on using 3D visualizations to convey information, because education systems do not focus on training people in 3D image comprehension skills, and because a skill as basic as spatial ability might be preventing many people from entering professions that now use or need 3D visualization comprehension skills, this research focuses on finding out how people understand 3D visualizations and what problems they might have with their comprehension based on specific properties of the visualization. Overall, we feel that this work can contribute to the design of comprehensible 3D information visualizations. This paper is organized as follows: In the next section, we justify the importance of this work, explain how it is related to information visualization, and connect it to other research that has already been carried out in this area. This is followed by an explanation of our research methods. Then we present our results along with supportive data for our findings and a discussion of these results. Finally, we summarize our work and present our conclusions. Background and Related Work Information Visualization Interest in information visualizations has increased both in terms of computer science investigation into the software needed to create information visualizations and as an area of research focused on how humans can benefit from such visualizations in the fields of information science, cognitive psychology, computer science, and human– computer interaction, especially in the past 10 years (Bederson & Shneiderman, 2003; Card et al., 1999; Chen, 2004). A number of software tools for visualizing information have been developed so that people can construct their own visualizations. One of the good examples is Many Eyes, which is a data visualization tool developed by IBM (Armonk, NY). Many Eyes lets people use either existing data sets provided by Many Eyes, or their own data sets to produce graphic representations such as charts, graphs, tag clouds, and diagrams. Another example is Tableau, which is a data visualization software that allows creating different kinds of visualizations and sharing them easily on the web. Another information visualization software, Datawatch, even lets users visualize data from streaming data. The growing interest in information visualization becomes more evident when the number of hits per decade for the search term “information visualization” is counted in Google Scholar (Julien, Leide, & Bouthillier, 2008). When search was limited to publications from 1980 to 1990, there were 1,060 hits. From 1990 to 2000, there were 4,690 hits, and from 2000 to 2010, the hit rate increased to 18,400. In information science, there exists a growing number of experimental studies that develop or propose information visualization systems or test the usefulness and effectiveness of such systems that are designed to support a user’s interaction with information, such as organizing, browsing, searching, and saving information. For instance, Eick (1994) developed a software tool called SeeSoft, which visualizes statistics associated with texts by using colored rows within columns. This system allows users to browse long lines of text including a technical paper, records in a database, or a literary corpus at a glance. Studies also focus on the effectiveness of using information visualization in information retrieval (Sebrechts, Vasilakis, Miller, Cugini, & Laskowski, 1999). For instance, Hoeber and Yang (2009) examined how HotMap, a system developed to visualize and interactively manipulate web search results, supports the users as they evaluate and explore search results for vague queries. HotMap represents the query term frequencies in the form of a color code on a heat scale in which multiple occurrences of a query term are represented as a dark red color while fewer occurrences are represented by progressively lighter shades of red, orange, and yellow. In their study, the researchers found an increase in speed and effectiveness and a reduction in missed documents when comparing HotMap to the list-based presentation used by Google. Participants also showed positive attitudes toward the HotMap interface. A screen shot of HotMap is presented in Figure 1a. Hearst (1995) developed a system called TileBars, which visualizes the length, query term frequency, and query term distribution of each document in the search results. She states that her visualization interface allows users to do a quick scan of the searched documents and supports users making judgments about the relevance of the documents (p. 59). 2 JOURNAL OF THE ASSOCIATION FORFOR INFORMATION SCIENCE ANDAND TECHNOLOGY—•• 2015 2016 1034 JOURNAL OF THE ASSOCIATION INFORMATION SCIENCE TECHNOLOGY—May DOI:DOI: 10.1002/asi 10.1002/asi a. Hotmap: Hoeber and Yang (2009). b. Envision: Nowell, France, Hix, Heath, & Fox (1996). c. VIEWER: Berenci et al. (2000). FIG. 1. Examples of information visualizations. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] JOURNAL OFASSOCIATION THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 3 JOURNAL OF THE FOR INFORMATION SCIENCE AND TECHNOLOGY—May 20162015 1035 10.1002/asi DOI:DOI: 10.1002/asi Heimonen and Jhaveri (2005) also incorporated visualization into a search results interface. For each document their user retrieved, query term frequencies were displayed in the shape of a small document-shaped icon. In the user study of its usefulness, the participants exhibited positive attitudes toward this visualization technique (p. 882). Another system, Envision, also visualizes the search results by using icons that represent the various attributes of each document via icon shape, size, and color, as shown in Figure 1b (Fox et al., 1993; Nowell, France, Hix, Heath, & Fox, 1996). Similarly, Berenci, Carpineto, Giannini, and Mizzaro (2000) developed a system called VIEWER, which is an interactive ranking system that provides a graphical visualization of results. To be more specific, when the user enters a query term, VIEWER first forwards this to a selected search engine and then shows the results returned by the search engine. It also provides a graphical visualization of the results consisting of several horizontal bars, which are subqueries that can be formed with the original query term within the retrieved results (p. 251). This allows users to select subqueries and conduct a search within search results. In the user study, the authors found that when using VIEWER, the search results were better than using a search engine without VIEWER. In addition, the authors reported that VIEWER provided increased retrieval effectiveness as well as user satisfaction (p. 249). The screenshot of the VIEWER is displayed in Figure 1c. Although the aforementioned studies did not examine the use and the effect of 3D visualizations, they illustrate that 2D displays in information visualization greatly facilitate an individual’s interaction with information. Because 3D visualizations allow more relationships to be presented in the visualization, the movement in information visualization design is toward 3D visualizations. The next section discusses this trend. 3D Visualizations Three-dimensional visualizations have become more common in our daily lives with the increasing graphical capabilities of computers (Hegarty, 2010; Velez, 2009; Velez et al., 2005). A number of software tools are available to create 3D visualizations easily without the need for people to write programs. In addition, both the abundance of data and the need to understand much data has increased the use of 3D visualizations in many fields, which include architecture, astronomy, biology, dentistry, geology, engineering, and medicine. In information science, there have been attempts to utilize 3D visualizations in a variety of ways. They have been used in representing document similarity by arranging objects spatially in a virtual environment where each object represents a document in the database (Westerman, Collins, & Cribbin, 2005). For example, Leuski and Allan (2000) developed a system called Lighthouse, which presents the user a ranked list and a 3D visualization of similarities among documents (Allan, Leuski, Swan, & Byrd, 2001; Leuski & Allan, 2000), as shown in Figure 2a. Similarly, Rennison (1994) developed a system called Galaxy of News, which visualizes large databases of news articles. In this system, news articles are clustered based on the content, and users can browse and search databases in 3D information space by zooming and panning. Wise et al. (1995) also developed a system called ThemeScape, which visualizes large text documents. In this system, text contents are spatialized in 3D space, and the system allows users to see the visual thematic summary of the whole corpus while exploring the document space. In addition, 3D visualizations have been used in representing hierarchically structured information. For example, Robertson, Mackinlay, and Card (1991) developed Cone Trees, which uses 3D visualizations to show hierarchical information. In a similar vein, Jeong and Pang (1998) proposed Disc trees, which also visualizes large hierarchical information spaces. Rekimoto and Green (1993) developed Information Cube, which lets users intuitively browse, search, organize, and select a large amount of information in a 3D cube. Figure 2b displays Information Cube. Threedimensional visualizations have also been used in visualizing domain knowledge. Using visualizations in analyzing domain structure by conducting citation analysis and link analysis has been quite common in infometrics, bibliometrics, and webometrics, although mostly using 2D visualizations. However, some researchers worked on visualizing domain structure using 3D visualizations. Figure 2c is a landscape view of the author cocitation analysis map. In this map, the height of a bar is the number of citations for the author, and the lighter shading in the color spectrum on each citation indicates more recent citations (Börner, Chen, & Boyack, 2003; Chen & Paul, 2001). Thus, there have been studies as well as systems and tools developed that show how 3D visualizations can be used to convey information in various ways. However, what we know about how people comprehend these visualizations is still minimal. Difficulties in Comprehending 3D Visualizations Although 3D visualizations can provide rich information, many people experience difficulties in comprehending 3D visualizations. Hearst (1999) stated that “although intuitively appealing, graphical overviews of large document spaces have yet to be shown to be useful and understandable for users” (p. 274). Even in relatively simple visualizations, it has been found that people experience difficulties in judging the depth, size, and position of objects (Young, 1996). Fabrikant et al. (2010) examined how people interpret 3D visualizations by conducting an experiment with 23 undergraduate students. The researchers found that inexperienced participants had difficulty interpreting the spatial metaphor used for information visualization. Modjeska and Chignell (2003) compared the effect of desktop virtual reality in a 3D and 2.5D environment by investigating how participants perform when finding topics in the 3D and 2.5D environment. They found that people with low spatial ability 4 JOURNAL OF THE ASSOCIATION FORFOR INFORMATION SCIENCE ANDAND TECHNOLOGY—•• 2015 2016 1036 JOURNAL OF THE ASSOCIATION INFORMATION SCIENCE TECHNOLOGY—May DOI:DOI: 10.1002/asi 10.1002/asi a. Lighthouse: Leuski and Allan (2000). b. The information cube: Rekimoto and Green (1993) c. Author co-citation analysis map: Börner et al. (2003) FIG. 2. Examples of 3D visualizations in the information science field. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] JOURNAL OFASSOCIATION THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 5 JOURNAL OF THE FOR INFORMATION SCIENCE AND TECHNOLOGY—May 20162015 1037 10.1002/asi DOI:DOI: 10.1002/asi experienced more difficulties when performing tasks within virtual environments. They also found that all participants performed better in the 2.5D environment than the 3D environment. Westerman et al. (2005) also compared the effect of using 2D and 3D visualizations for information retrieval. Their study participants used a more focused approach when searching in the 3D environment and a more exhaustive approach in the 2D environment. The researchers reported that there were no significant differences in terms of the information retrieval effectiveness and efficiency in 2D versus 3D conditions. Powers and Pfitzner (2003), who examined various information visualization interfaces, also were skeptical about 3D visualizations, saying that “generally 3D graphical interfaces have proven to be ineffective” (p. 534). However, the researchers mentioned that there seems to be a general experience factor that helps users makes better use of complex interfaces such as the 3D interface. In a similar vein, Sebrechts et al. (1999) conducted a comparative evaluation of the effectiveness of text, 2D, and 3D interfaces in information retrieval and found that, while there were high interface learning costs for the 3D visualization, they decreased significantly with experience. These results show that although 3D visualizations can be difficult to comprehend, the ability to comprehend these 3D visualizations can be improved with experience and training. Previous Research on 3D Visualization Comprehension As we have previously stated (Oh et al., 2011), in the fields of geology and psychology the focus has been on spatial abilities and their relationship to 3D visualization comprehension skills. Velez (2009) examined what spatial abilities were correlated with 3D projection visualization comprehension. She reported that the accuracy of participants on the visualization comprehension test had a medium-high correlation with spatial visualization abilities. Alle (2006) conducted a study to examine and characterize the 3D spatial abilities of geology students by measuring their ability to comprehend the 3D structure of folded sedimentary rocks from surface structure visualizations. Alle found that students had different visual penetrative ability (VPA), which is the “ability mentally to penetrate the image of a structure” (Kali & Orion, 1996, p. 369) when solving geology problems. On one hand, some participants constructed a complete 3D mental model of the visualization quickly and drew the required cross-sectional elements based on the presented surface information. On the other hand, other participants were not able to associate the surface information of the geological structures with their interior. The participants’ attempts to draw cross-sections showed many common nonpenetrative errors. Other reported findings show that people exhibit large individual differences in comprehending spatial imageries, with some of these differences being based on the professions the people work in (Blajenkova, Kozhevnikov, & Motes, 2006; Kozhevnikov, Kosslyn, & Shephard, 2005). Velez’s (2009) study is one of the few studies that attempted to identify properties of the 3D visualizations that influenced people’s ability to create a mental picture of an object. She found that the problems that only participants with high spatial ability answered correctly were visualizations that had more edges and vertices, that is, these properties make the visualization more difficult to comprehend. Similarly, Cohen and Hegarty (2007b) conducted an experiment and investigated how participants solved simple 3D cross-section visualization problems using a Santa Barbara Solid Test they developed. Cohen and Hegarty also measured individual spatial abilities and found that it correlated with a participant’s performance on their multiple choice spatial ability test. They reported that oblique cutting planes were more difficult to comprehend than 3D visualizations with orthogonal cutting planes. Although they designed problems to represent some of the additional features being investigated in our study, the use of color to represent the various objects embedded in the solid objects used in the problems led to a presumed color advantage that aided problem solutions. The object used in Cohen and Hegarty’s study is displayed in Figure 3. Our research study differs from this work in that it does not come with pre-prepared hypotheses but, rather, is focused on generating possible hypotheses to test later in controlled experiments. It is a qualitative research rather than quantitative research study; something we feel is needed first to effectively explore what the underlying comprehension issues might be. In other words, before considering and testing possible hypotheses as to what may make comprehension more difficult or easier, we wanted to consider multiple possibilities and, thus, uncover potential visualization properties that affect comprehension which we might not have thought of. To do this, we employed a think-aloud FIG. 3. Santa Barbara Solids Test figures (Cohen & Hegarty, 2007b). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] 6 JOURNAL OF THE ASSOCIATION FORFOR INFORMATION SCIENCE ANDAND TECHNOLOGY—•• 2015 2016 1038 JOURNAL OF THE ASSOCIATION INFORMATION SCIENCE TECHNOLOGY—May DOI:DOI: 10.1002/asi 10.1002/asi method that allowed us to learn the cognitive processes participants were going through as they tried to solve each visualization problem. The research on verbal protocols indicates that people can effectively report what they are thinking if (a) they are not performing routine problem solving and (b) their cognitive load is not too high so that verbalization does not interfere with the thinking processes. Overall, the aforementioned studies investigated and reported a number of useful and interesting findings related to 3D visualization comprehension. However, how and why people experience difficulty in comprehending 3D visualizations is still not understood. Thus, we have conducted our qualitative research study to stand back and determine if various visual properties of 3D visualizations might make comprehension easier or more difficult. Having such knowledge might help us figure out the mechanism behind such comprehension. Even if we cannot find the mechanism, at least we will start to understand what properties of visualizations affect comprehension. The reader should note that this study is only a hypothesis generation study. We do not claim that the properties we have uncovered in this research are correct, only that it would be interesting research to investigate these properties further. textbooks. For the experiment, these visualizations were redrawn in black and white using textures to represent the colors and also modified slightly to make it easier for participants to redraw portions of the diagrams. (b) Characteristics of collected visualization diagrams were analyzed using five standard geologic properties: number of layers, number of faults, number of folds, whether layers were orthogonal to the planes of reference, and the angle of the layers if not orthogonal to the x–y plane the visualization was presented in. This was done to ensure a wide representation of properties that are typically used in these visualizations. (c) Then cross-section/slice visualization problems that participants were to solve in the experiment were developed (see Figure 4). Last, (d) The slice visualization problems were ordered from easiest to hardest for presentation to participants by asking seven experts and novices to solve the visualization problems and rank order the problem set by difficulty. Via this process, a set of 18 3D cross-section visualization problems ordered by increasing difficulty were created. Examples of easy and difficult 3D visualization problems are displayed in Figure 4. Method Research Questions a As this study was exploratory in nature, we did not have hypotheses regarding which visual properties are easier or more difficult to comprehend. The two primary research questions were the following: RQ1: What are the visual properties of 3D visualizations that make it more difficult for users to understand the information being presented? RQ2: What are the visual properties of 3D visualizations that make it easier for users to understand the information being presented? b Participants Eleven undergraduate students at Rutgers University were recruited as participants by posting notices on the campus and via recruitment in classes. They were monetarily rewarded for their participation and were mostly from the communication or engineering departments. We focused on recruiting half of our participants from nonscience disciplines and being female since such individuals are likely to have lower spatial abilities. Among 11 participants, six were male and five were female. Generation of 3D Visualization Diagrams For the study, a set of 18 3D visualization diagrams were generated. We generated these diagrams as follows: (a) a set of 35 representative 3D visualizations were collected from introductory geology and earth science FIG. 4. (a) Easy and (b) difficult 3D visualization problems. JOURNAL OFASSOCIATION THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 7 JOURNAL OF THE FOR INFORMATION SCIENCE AND TECHNOLOGY—May 20162015 1039 10.1002/asi DOI:DOI: 10.1002/asi Procedure Analysis In the experiment, participants were asked to visualize the internal structure of 18 3D visualization diagrams by drawing what the cut face of the visualization would look like if it were cut along the dotted lines shown in Figure 4. Each 3D visualization problem was drawn on the top part of an individual sheet of paper and given to participants in an order of increasing difficulty. Then the participants were asked to draw the cut plane of the 3D visualizations on the bottom part of the sheet of paper. In this process, a thinkaloud protocol, which asks participants to verbalize their thoughts was used (Ericsson & Simon, 1980; Van Someren, Barnard, & Sandberg, 1994) to examine the cognitive processes and difficulties underlying visualization comprehension. Thus, participants were asked to verbalize their thoughts while solving 3D visualization problems. Each session lasted for approximately 1 hour per participant, and each session was videorecorded. Researchers asked participants to stop when it took more than an hour even when the participants did not finish solving all of the problems. The videorecordings were analyzed by focusing on participants’ verbal protocols as well as their gestures and drawings. While analyzing data, A grounded theory technique (Strauss & Corbin, 1990) was employed to generate hypotheses. To be more specific, we first transcribed each of the videorecordings, and as we analyzed the transcription, we identified emerging patterns that eventually formed a set of hypotheses as to what properties of the visualizations made them difficult or easy to comprehend. We also noted the support for each hypothesis. Potential disconfirming evidence was also noted and searched for. This way, some hypotheses were supported with verbal and gesture evidence, while some of them were either modified or eliminated. Videorecordings were transcribed and annotated using a video annotation tool, Transana (http://www.transana.org/). In addition, while identifying emerging patterns that were observed multiple times across videorecordings, a table was developed. A portion of this table is presented in Figure 5. Each videorecording was viewed more than five times by multiple researchers to generate the hypotheses. FIG. 5. Table developed while analyzing data. 8 JOURNAL OF THE ASSOCIATION FORFOR INFORMATION SCIENCE ANDAND TECHNOLOGY—•• 2015 2016 1040 JOURNAL OF THE ASSOCIATION INFORMATION SCIENCE TECHNOLOGY—May DOI:DOI: 10.1002/asi 10.1002/asi FIG. 7. Cut. This is illustrated as the dotted line through the diagram. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] FIG. 6. Planes of references. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] The authors note that the analysis of a verbal protocol is an unusual use of grounded theory which is usually applied to sociological studies. Verbal protocols have typically been used in studies of problem solving and also to determine user problems with interfaces. Most prior research conducted on understanding how people comprehend visualizations has been carried out with controlled experiments. Visualizations, however, are complex and require multiple combined skills to understand. We therefore felt that it was more appropriate to begin by generating multiple hypotheses based on observing individuals struggle with problem solutions before beginning any controlled experiments. We used a grounded theory approach to focus on those hypotheses that occurred frequently, but we also added controls to the study, which is not typically done, in order to ensure that we had a representative population. FIG. 8. Flat/planar layers. These are illustrated as the continuous lines in the diagram. FIG. 9. Nonplanar layers. These are illustrated as the continuous lines in the diagram. Definitions The operational definitions of key terminologies in this study are as follows. 1. 3D visualization problems: 18 3D visualization diagrams that are given to the participants to visualize. 2. Planes of reference: A viewer’s automatically assumed x, y, and z coordinate system (Heo & Hirtle, 2001). The x and y axes form one plane. The x and z axes another with the y and z axes the third plane (Figure 6). Because the diagram is presented as a block, the viewer automatically assumes that the shown x, y, and z axes are the coordinate system in use. 3. Cut: Plane that slices 3D visualization into two pieces (Figure 7). It is often referred to as a cross-section in different fields (Alle, 2006; Cohen & Hegarty, 2007b; Hegarty, Keehner, Khooshabeh, & Montello, 2009; Velez, 2009). 4. Flat/planar layers: Continuous features inside the 3D visualization that are parallel to each other or to the planes of reference (Figure 8). 5. Nonplanar layers: Continuous features inside the 3D visualization that are not parallel to each other or to the planes of reference (Figure 9). Results and Discussion The results from our videotape analysis show that participants often struggle with visualizing the internal structure of 3D visualizations. Participant 5 described his/her difficulty as “I’m lacking some sort of understanding or skill or whatever that enables me to see what these would look like from the inside. It’s like a . . . like a . . . how a child can understand another person’s point of view. It’s almost like that. Like I can’t look at this in any view except what is in front of me.” An in-depth analysis of the results showed that the following visual properties made problem solving more difficult for participants: (a) a cut that was not parallel or JOURNAL OFASSOCIATION THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 9 JOURNAL OF THE FOR INFORMATION SCIENCE AND TECHNOLOGY—May 20162015 1041 10.1002/asi DOI:DOI: 10.1002/asi perpendicular to at least one plane of reference (see Figure 10a), (b) nonplanar layers that formed the visualization including curved layers (see Figure 10b) and v-shaped layers (see Figure 10c), and (c) mixed combinations of layers (both parallel and nonparallel to a plane of reference; see Figure 10d). On the other hand, (a) cuts that were perpendicular or parallel to the diagram’s planes of reference (see Figure 11a), (b) internal features that were flat/planar and parallel to one plane of reference (see Figure 11b), and (c) internal homogeneous layers, consisting of either only curved or flat/planar layer formations that were parallel to each other were easier to comprehend (see Figure 11c). We could not find sufficient evidence for the effect of the shape of the visualization, the effect of layers being at an angle to the planes of reference, and the location of the cutting plane in the visualization, for example, either close to the front or back of the visualization. The paragraphs below document support for each of the properties that were found to cause difficulties. a b c d FIG. 10. Properties of 3D visualizations that made problem solving more difficult. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] a b Angled Cuts Participants found it difficult to form a mental image of the patterns that would appear on the cut plane when the cut was at an angle to the planes of reference. This finding is consistent with prior research studies (Cohen & Hegarty, 2007a, 2007b; Pani, Jeffres, Shippey, & Schwartz, 1996; Pani, Zhou, & Friend, 1997). In particular, Cohen and Hegarty (2007b) reported that participants in their study consistently had difficulties in forming a mental image of the cut plane across three different types of 3D visualizations— simple, joined, and embedded—when the cut was at an angle to the planes of reference. While visualizing 3D visualization problems with angled cuts, participants often directly mentioned about the difficulty caused by the angled cuts. For example, Participant 8 said that “This thing is a little hard because of the angle,” and “I think I have troubles with the angle, that’s the problem for me.” Participant 7 also mentioned that “The difficult part for me is to visualize and imagine when the angle goes down,” which clearly shows the difficulty caused by the angled cuts. While visualizing, participants often paused, pointed at the cut with a finger or pen, and used hands to virtually cut visualization problems at an angle to the plane of reference by covering part of the figure. For example, while visualizing Figure 12a, participant 5 said, “It is cutting it this way [pointing at the cut with the pen], sort of [pausing]. Hmm . . . not actually sure [pausing]. I don’t know . . . uh . . . [pointing at the cut with the pen again] I can’t really tell it’s cutting at a diagonal,” indicating that the participant was having difficulty in visualizing the cut plane because of the angled cut. In addition, some participants tried to ignore the angle in the cut and regard it as a cut that was parallel or perpendicular to a plane of reference, that is, a totally vertical or horizontal cut. For instance, while visualizing the cut plane of Figure 12a, Participant 9 said, “I feel like this is just going to be the face. ‘Cause if you bring it (cutting plane) up, the way its cut will just make it similar to the first part, to the flat face.” Here, the participant was trying to ignore the angle, and regard the angled cut as a cut that is parallel to the x, y plane and perpendicular to the y, z plane. Figure 12 shows the 3D visualization problem with an angled cut (Figure 12a), the correct answer (Figure 12b), and the examples of wrong answers given by participants (Figure 12c,d). c FIG. 11. Properties of 3D visualizations that made problem solving easier. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] 10 1042 JOURNAL OFOF THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 20152016 JOURNAL THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—May DOI: 10.1002/asi DOI: 10.1002/asi a b c d FIG. 12. An example of a 3D visualization problem with an angled cut and its solutions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] FIG. 13. An example of a 3D visualization problem with an angled cut. In a similar vein, before visualizing Figure 13, which has the cutting plane that is cut with an angle, Participant 6 tried to imagine the cut that is parallel to the x, y plane and perpendicular to the y, z plane, saying that “If this slice is totally horizontal, I would have five layers.” Figure 13 shows the 3D visualization problem with an angled cut. When trying to figure out the thickness of each layer, participants had to consider more aspects of the solution when the 3D visualization problem has an angled cut. With a parallel cut, the thickness is the same as seen on one of the faces, but when the cut is angled, there is an added problem of calculating the thicknesses via projection. As the thickness of each layer that needed to be drawn in the cut plane is different from what is shown on the surface of the visualization, participants often tried to figure out whether a certain layer was going to be thicker or thinner than what was shown on the surface of the visualization as a result of angled cut. For example, while solving the problem in Figure 14a, participant 6 said, “Even when I take a cross section at an angle, it is going to have three layers, but the thickness of layers is going to be changed.” Figure 14 shows the 3D visualization problem with an angled cut (Figure 14a), the correct answer (Figure 14b), and the answer given by Participant 6 (Figure 14c), which was correct. Figuring out which part of the layer will be included in the cutting plane also required the participants to consider more aspects of the solution when visualizing a 3D visualization problem with an angled cut. When solving these problems, sometimes the thicknesses of layers were wrong, a layer was shown when it should be gone, or a layer was gone when it should be shown, which indicated the difficulty. For instance, while visualizing Figure 15a, Participant 3 said, “I’ll not see this layer [pointing at the last layer in the 3D visualization problem]. So I’ll see just one thing at the end which is these dots from C to D, and the other side here.” Figures 15 and 16 show the 3D visualization problems with angled cuts (Figures 15a and 16a), the correct answers (Figures 15b and 16b), and the wrong answers given by the participants (Figures 15c,d and 16c,d). JOURNAL OF ASSOCIATION THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 2015 1043 11 JOURNAL OF THE FOR INFORMATION SCIENCE AND TECHNOLOGY—May 2016 DOI: 10.1002/asi DOI: 10.1002/asi a b FIG. 14. An example of a 3D visualization problem with an angled cut and its solutions. a b c d FIG. 15. An example of a 3D visualization problem with an angled cut and its solutions. Some participants made mistakes as they thought the cut plane would not be a square because of the angle in the cut. For example, while visualizing Figure 14a, Participant 7 drew the cut as a parallelogram, while saying that “It’s a diagonal slice, so probably the square I will have to draw is also diagonal.” Participant 3 also asked “Slice is always a rectangle, right?” showing confusion caused by the angled cut. Figure 17 shows the examples of wrong answers that were drawn as parallelograms. As described, participants often drew wrong answers to this problem compared to a similar 3D visualization problem with an orthogonal cutting plane. Wrong answers included failing to adjust the thickness of layers, as shown in 12 1044 c Figure 12c,d, not drawing layers that would actually be included in the solution, or drawing layers that would not be included in the solution, like Figure 16c,d, and drawing a cutting plane that is parallelogram, as displayed in Figure 17a,b. It seems that the primary reason why participants are having more difficulties when the cut is angled to the plane of reference than when not is because the direction of the cut is different from the planes of reference, that is, a viewer’s automatically assumed x, y, and z coordinate system (Figure 6). In fact, previous studies showed that transforming objects mentally with axes oblique to the environment are more difficult to perform than transforming JOURNAL OFOF THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 20152016 JOURNAL THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—May DOI: 10.1002/asi DOI: 10.1002/asi a b c d FIG. 16. An example of a 3D visualization problem with an angled cut and its solutions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] a objects with orthogonal axes to the environment (Cohen & Hegarty, 2007b; Pani et al., 1997). There are two possible reasons for this. First, as indicated by the need to do projection mapping to determine the thickness of the layers, the cognitive load for solving an angled cut problem is higher. Second, because most of the world we live in uses rectangular coordinates, it is likely that we do not have automatic mappings to prior solutions or approximate solutions to these problems. b Nonplanar Layers FIG. 17. Examples of wrong answers when visualizing 3D visualization problems with an angled cut. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] Nonplanar layers in the 3D visualizations including curved layers and v-shaped layers also were more difficult to visualize for the participants than flat/planar layers. In the case of curved layers, while visualizing Figure 18a, Participant 5 said “This (left side of the diagram) is curvy and this (right side of the diagram) is straight, so what will the inside look like? I don’t know,” illustrating the confusion caused by the curved layers. Figure 18 shows the 3D visualization problem (Figure 18a), the correct answer (Figure 18b), and the answer given by Participant 3 (Figure 18c), which was correct. Similarly, while visualizing Figure 18a, Participant 8 mentioned that “This one was more difficult because it didn’t have straight lines any more, the curve is coming from the inside of the slice. I don’t know why, but I can’t visualize or even think about this arc.” JOURNAL OF ASSOCIATION THE ASSOCIATION INFORMATION SCIENCE AND TECHNOLOGY—•• 2015 1045 13 JOURNAL OF THE FOR FOR INFORMATION SCIENCE AND TECHNOLOGY—May 2016 DOI: 10.1002/asi DOI: 10.1002/asi a b FIG. 18. An example of 3D visualization problem with curved layers and its solutions. a d c b e c f FIG. 19. An example of a 3D visualization problem with curved layers and its solutions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] Participants often paused and did not say or do anything for a while when they were confused. For example, while visualizing Figure 19a, Participant 6 said, “There [pausing] are curved layers. So this is going to be . . . [pausing]. I see [pausing], alright. Mmm . . . [pausing],” which illustrates the difficulty she/he was having because of the curved layers. Figure 19 shows the 3D visualization problem (Figure 19a), the correct answer (Figure 19b), and the wrong answers given by participants (Figure 19c–f). Common wrong answers caused by curved layers included drawing flat/planar layers when they actually had to draw curved layers, like the solution for the visualization problem displayed in Figure 19c–f or drawing curved layers (Figure 20) when they actually needed to be straight (Figure 14b). Participants found it extremely difficult to visualize what would appear on the cut plane when the layers were v-shaped. Participants were often lost while solving visualization problems with v-shaped layers such that they did not do anything for a long time, and even gave up attempting to visualize the cutting plane. For instance, while trying to solve the visualization problem shown in Figure 21a, Participant 5 said “[Not doing anything for a long time] I don’t 14 1046 FIG. 20. An example of a wrong solution to the 3D visualization problem with curved layers. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] know. I really don’t know, I just . . . can’t see . . . Yeah, I don’t, I don’t know if it would actually be like this. Yeah, I don’t really know. I don’t know.” Wrong answers included not drawing anything at all or drawing parallel layers in the cutting plane when they were actually v-shaped, such as the solution to the visualization problem presented in Figure 21c,d. Figure 21 shows the 3D visualization problem JOURNAL OFOF THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 20152016 JOURNAL THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—May DOI: 10.1002/asi DOI: 10.1002/asi a c b d FIG. 21. An example of a 3D visualization problem with v-shaped layers and its solutions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] with v-shaped layers (Figure 21a), the correct answer (Figure 21b), and the wrong answers given by the participants (Figure 21c,d). However, when the layers were flat/planar, participants found the problems easy, saying “This is pretty simple” (Participant 9 working on Figure 4a). For most of these cases, participants drew the cutting plane correctly without stopping. We hypothesize that the primary reason for the difficulty is similar to the reason for the angled cut. To be more specific, nonplanar layers make them difficult to comprehend because the visualization requires a projection of a complex object on to the cutting plane, adding to the cognitive load of the visualizer. In particular, in the case of the curved layers, it was difficult for participants to figure out whether the images of the curved layers in the cut plane of the 3D visualization problems would be curved or flat/ planar. Mixed Combinations of Layers Mixed combinations of curved and flat/planar layers were also more difficult to visualize than homogeneous layers, which contained either only curved or flat/planar layer formations. As previously mentioned, visualization problems were given to participants in an order of increasing difficulty, and visualization problems with mixed combinations of curved and flat/planar layers were problem numbers 15, 16, and 18 among the 18 visualization problems, showing that they were already considered to be relatively difficult visualization problems among all of the given problems. Thus, only a few of our participants solved these problems, as most of the participants stopped even before reaching these problems in the allotted hour. Sometimes participants who reached these problems gave up. For instance, while solving Figure 22a, Participant 1 said “I don’t understand this one at all (laughing) . . . I’m trying to understand how to visualize this . . . I don’t know, I really can’t. I give up on this one,” indicating the difficulty. Figure 22 shows the 3D visualization problem with mixed combinations of layers (Figure 22a), the correct answer (Figure 22b), and the wrong answers given by the participants (Figure 22c,d). In solving these visualization problems, participants often exhibited difficulties in figuring out whether they should draw straight or curved lines when visualizing certain layers in the cutting plane. For instance, while visualizing Figure 23a, Participant 6 said “This is going to be straight? Can I see the curve over here [pointing to the cut with an index finger]? Yeah, there should be a curve over there for the dense layer. There should be a curve for the next layer, too?” Similar to the visualization problems with v-shaped layers, participants were often lost while solving visualizations with a mixed combination of curved and flat/ planar layers. Participant 5 said, “Just as I start to think I’m getting somewhere, I completely lose the image in . . . in my brain. And then I can’t, think of, what it would look like.” Wrong answers from these visualization problems include drawing flat/planar layers in the cross-section when they actually were curved, as in the case of Figure 23c, and failing to draw the layers in their correct directions or size, as in the case of the visualization problem shown in Figure 23d. Figure 23 shows the 3D visualization problem with mixed combinations of layers (Figure 23a), the correct answer (Figure 23b), and the wrong answers given by the participants (Figure 23c,d). On the other hand, participants found it easier to visualize 3D visualization problems with homogeneous layers than mixed combinations of curved and flat/planar layers. While visualizing Figure 24a, which has only curved layers, Participant 1 said, “I could go layer by layer, it’s pretty straightforward.” Similarly, while visualizing Figure 24b, Participant 8 said, “I didn’t have problems here because it has the same shape for each layer, they were all rectangles, so it was much more consistent and it didn’t have different patterns,” indicating that consistency of layers, that is, homogeneous layers made the cut easier to visualize. In this case we attribute the problems with the nonhomogeneous layers to the additional cognitive load caused by the mix of layer types. Three-dimensional visualization problems with homogeneous layers are presented in Figure 24. JOURNAL OF ASSOCIATION THE ASSOCIATION INFORMATION SCIENCE AND TECHNOLOGY—•• 2015 1047 15 JOURNAL OF THE FOR FOR INFORMATION SCIENCE AND TECHNOLOGY—May 2016 DOI: 10.1002/asi DOI: 10.1002/asi a c b d FIG. 22. An example of a 3D visualization problem with mixed combinations of layers and its solutions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] Conclusion This qualitative research study used grounded theory to find out which properties in 3D visualizations made them easy or difficult to comprehend. Our initial findings suggest that cross-sections made at an angle to the planes of reference of the visualization, curved layers, v-shaped layers, and mixed combinations of curved and flat/planar layers made visualizations more difficult for our participants to comprehend the inner structure of the 3D visualizations. In contrast, cross-sections that were parallel or perpendicular to the planes of reference of the visualization, flat/planar layers, and homogeneous layers created easy to solve visualizations. This result shows that the direction of the cut face of the 3D visualization influences the easiness and difficulty of the comprehension of the internal structure of 3D visualizations. In particular, this research showed that when either the direction of the cut or the layers is different from the assumed planes of reference, a reorienting of a participant’s thinking to new planes of reference was required, which made it difficult for the participants to comprehend the 3D visualizations. By choosing a set of 3D visualizations from introductory geology and earth science textbooks, we were able to separate the domain knowledge from the visualization; that is, 16 1048 we did not ask any questions about what the geology layers meant, but rather only to revisualize the visualization from a different perspective and to draw this revisualization. We did not do this with information visualizations because many of those currently in use make it very difficult to separate the domain knowledge from the visualization. We also did not do this because many of the 3D information visualizations are much more complicated than the geological ones because they require a visualizer to establish their own planes of reference and because they usually are animated; that is, they can be rotated, zoomed in and out of, and viewed from different perspectives. We felt that these types of capabilities would have made our study problems too difficult for our participants and not have enabled us to obtain the information we did. This work makes a direct contribution to the understanding of the aspects of 3D visualizations that reduce or enhance their effectiveness. The understanding of these aspects contributes to the design and use of any 3D visualization in a way that helps users to better interact with information. Most directly, the research connecting 3D visualization comprehension to spatial ability in the study of scientific visualizations can be applied to information visualization as well, since any 3D visualization, including scientific visualizations and information visualizations, JOURNAL OFOF THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—•• 20152016 JOURNAL THE ASSOCIATION FOR INFORMATION SCIENCE AND TECHNOLOGY—May DOI: 10.1002/asi DOI: 10.1002/asi a b c d FIG. 23. An example of a 3D visualization problem with a mixed combination of layers and its solutions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] a b FIG. 24. Three-dimensional visualization problems with homogeneous layers. requires spatial ability for comprehension. In addition, since there is relatively little difference in the visual properties forming both we can expect similar comprehension problems. More subtle in nature, our research suggests that individuals approach virtual visual scenes with an implied 3D frame of reference (what we call our planes of reference) deciding what is the up–down direction, the left–right direction, and the back–front direction. They then solve problems based on their chosen orientation to this frame of reference or viewpoint and have difficulties with features in the visualization that are not parallel or perpendicular to their viewpoint. The concrete nature of science visualizations often helps a viewer construct a frame of reference. Information visualizations typically do not. Further, they often allow a viewer to rotate the visualization and/or perform the rotation for the viewer. Because a viewer does not have any knowledge of what might make a good viewpoint, they are unlikely to achieve a rotation that will help in comprehending the visualization, making the comprehension difficulties worse. Although future research is necessary to form design recommendations, our work suggests possible mechanisms for generating more comprehensible 3D information visualizations by establishing an easily recognized frame of reference and then by avoiding visual properties that do not have simple relationships to this frame of reference. JOURNAL OF ASSOCIATION THE ASSOCIATION INFORMATION SCIENCE AND TECHNOLOGY—•• 2015 1049 17 JOURNAL OF THE FOR FOR INFORMATION SCIENCE AND TECHNOLOGY—May 2016 DOI: 10.1002/asi DOI: 10.1002/asi This study used a qualitative research method to generate hypotheses as to which properties make 3D visualizations easy or difficult to comprehend. This was a an appropriate method; however, it has limitations in that there were a small number of participants in the study. In addition, as the study was conducted with no preconceived beliefs as to what might cause comprehension difficulties, it was not conducted in a controlled manner, nor were the problems selected for specific properties. Therefore, it is possible that the 18 3D visualization problems used in our study may not comprehensively covered all the problems that might cause difficulty. Also, because many of the 3D visualization problems had multiple properties, there is a possibility that those properties interacted with each other. Lastly, the cross-section was an artifact of the experiment, that is, it created a problem to solve, so in some sense, talking about the angle of the cut is also representative of this artifact. However, an argument can be made that a cross-section represents a mental interpretation of the interior of a visualization and, thus, is a useful way of determining if a person truly understands the entire visualization. In the future, the plan is to run confirmation studies that test whether the visual properties that we found really do make 3D visualizations easy or difficult to comprehend. Controlled experiments will be conducted so that whether a certain property makes it easier or more difficult can be accurately measured. The first controlled experiment will test whether the direction of the cut makes comprehension of the internal structure of 3D visualizations easy or difficult by asking participants to find a correct cut face of the 3D visualizations when they are cut in various directions. In the second controlled experiment, whether the shape of the layers affects 3D visualization comprehension will be tested by asking participants to find a correct cut face of the 3D visualizations when 3D visualizations have various types of layers. In addition, although we only used blockshaped 3D visualizations, we plan to investigate whether the overall shape of the 3D visualizations will make it easier or harder to comprehend the internal structure of the 3D visualizations. Because interest in 3D information visualizations are growing in number and 3D information visualizations are getting more sophisticated and complex, we believe that discovering specific visual properties such as the direction of the cut, the shape of the inner layers, and the outer shape of the 3D visualizations that make comprehending 3D visualization easier or more difficult will significantly contribute to the design of 3D information visualizations and, also, the development of training programs that facilitate users’ understanding and interaction with information. Acknowledgments This study was part of the research project, which was supported by a grant from the National Science Foundation (#0753176). 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