Regression in high school: An empirical analysis of Spanish textbooks Marı́a Gea, Carmen Batanero, Pedro Arteaga, José Miguel Contreras, Gustavo Cañadas To cite this version: Marı́a Gea, Carmen Batanero, Pedro Arteaga, José Miguel Contreras, Gustavo Cañadas. Regression in high school: An empirical analysis of Spanish textbooks. Konrad Krainer; Naďa Vondrová. CERME 9 - Ninth Congress of the European Society for Research in Mathematics Education, Feb 2015, Prague, Czech Republic. pp.658-664, Proceedings of the Ninth Congress of the European Society for Research in Mathematics Education. <hal-01287065> HAL Id: hal-01287065 https://hal.archives-ouvertes.fr/hal-01287065 Submitted on 11 Mar 2016 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Regression in high school: An empirical analysis of Spanish textbooks María M. Gea, Carmen Batanero, Pedro Arteaga, J. Miguel Contreras and Gustavo R. Cañadas University of Granada, Faculty of Education, Granada, Spain, [email protected] The aim of this study was to analyze the presentation of regression in Spanish school textbooks aimed at Social Sciences students. In a sample of eight textbooks we analyzed the problem-fields and suggested procedures, as well as the associated concepts, which are classified according to the way they are introduced and the properties associated with them. Our results suggest that these textbooks mostly reduce regression to lineal regression, present many properties at an operational level without deep discussion of their meaning, and there is a great variety between the textbooks. Some suggestions to improve the presentation of this topic are included. Keywords: Regression, textbooks, high school, mathematical objects. INTRODUCTION Correlation and regression are fundamental statistical ideas, due to their usefulness to model phenomena in different fields (Engel & Sedlmeier, 2011). Previous research is mainly focused on students understanding of correlation and describes several misconceptions (Estepa & Batanero 1995; Estepa, 2008; Zieffler & Garfield, 2009). There has not been, however, much interest in the way the topic is taught or presented in the textbooks, even although textbooks are important educational tools. From the official curricular guidelines to the teaching implemented in the classroom, an important step is the written curriculum reflected in textbooks (Herbel, 2007). The selected textbook is an important part of teaching and learning mathematics at secondary and high school level, since the presentation of the topics and the problems proposed provide the main basis of why the topic is taught (Shield & Dole, 2013). Moreover, in other mathematical topics, textbooks receive increasing attention from the international community; see for example Fan, and Zhu (2007). CERME9 (2015) – TWG05 The aim of this research was to analyze the presentation of regression in high school Spanish textbooks aimed at Social Sciences students. It is part of a wider project where we compare the way in which correlation and regression are presented in the textbooks in Spain. Due to length limitation we only present here a part of our analysis; complementary results were published in (Gea, Batanero, Cañadas, & Contreras, 2013), and (Gea et al., 2014). THEORETICAL BACKGROUND According to Rittle-Johnson, Siegler and Alibali (2001) conceptual and procedural knowledge are the two extremes in a continuous; conceptual knowledge is more flexible and includes the implicit or explicit understanding of a domain and its interrelationships. Sfard (1991) suggests that a concept is a construct which corresponds to the mathematical universe and distinguishes two types of definitions of the same: structural (where concepts are introduced by describing their essential conditions or properties) and operational (when it is described by a formula) definitions. In our analysis we will take into account both types of definitions and will also describe in detail the properties associated with regression. The division between conceptual and procedural knowledge is analyzed in the onto-semiotic approach to mathematics education (Godino & Batanero, 1998; Godino, 2002) where mathematical activity plays a central role. Knowledge in this framework is modelled in terms of systems of practices, in which different types of objects intervene when a subject faces the solution of a given problem. “Object” is understood in a broad sense for any entity which is involved in mathematical practice and can be identified separately. More specifically in our analysis we will consider the following types of mathematical objects: problem-situations; procedures; concepts and properties. 658 Regression in high school (María M. Gea, Carmen Batanero, Pedro Arteaga, J. Miguel Contreras and Gustavo R. Cañadas) Previous research In spite of the relevance of the topic, previous research suggests poor results in people’s understanding of correlation. For example, Chapman and Chapman (1967) described illusory correlation where people are often guided by their theories rather than by data when estimating correlation. Estepa (1994) defined several misconceptions including the deterministic conception, where the student only considers whether functional dependence exists. Sánchez Cobo (1999) described students’ failures in ordering correlation coefficients and in interpreting the relationship between the correlation coefficient and the slope of the regression line. As regards research on textbooks, Sánchez Cobo (1999) classified the definitions of concepts presented in 11 textbooks published in the period 1977–1990 as procedural, structural or a mixture of both. Lavalle, Micheli and Rubio (2006) analyzed the concepts and procedures included in 7 high school textbooks from Argentina. As stated in the introduction, this paper is part of a more comprehensive project. In (Gea et al., 2013) we analyze the problem-situations used to contextualize correlation and regression in eight Spanish textbooks and in (Gea et al., 2014) the symbolic, verbal and graphical language used in this topic. The current paper complements these publications with the analysis of concepts, properties and procedures included in the study of regression in the same textbooks, which were not included in these papers. In each of these types of objects we analyzed all the categories that are relevant for the teaching of the topic. METHOD The sample consisted of eight mathematics high school textbooks aimed at Social Sciences students and that were published just after the current curricular guidelines were introduced (MEC, 2007). These particular textbooks have a wide diffusion in Spain due to the publishers’ prestige and are still used in the schools (See Appendix). We performed a content analysis (Neuendorf, 2002) of the chapters devoted to correlation and regression with the following steps: a) following an inductive and cyclic procedure we first categorized all the different mathematical objects explicitly or implicitly included in the chapter (the different types of problems, concepts, properties and procedures); b) For each of them we analyzed the way in which they are described or used in the textbooks. In this paper we only describe the mathematical objects linked to regression; as the remaining results have been published elsewhere (Gea et al., 2013; 2014). PROBLEMS AND PROCEDURES Anthony and Walshaw (2009) reported on the different types of tasks that have been analysed in mathematics education research, that include problems focused on specific mathematical content; problems that promote mathematical modelling; others that require discussion of aspects that vary; those that ask students to interpret and criticise data and those that prompt sense making and justification of thinking. Gea and colleagues (2013) identified two main types of problems in the study of regression in which one of several of the above type of tasks may be combined: a) Fitting a model to the data requires sense making and criticism of data; mathematical modelling, discussion of variation and justification of thinking, b) Using the model to predict a value of the dependent variable is a more computational type of task; however, much of the time judgement of the goodness of fit involves criticism, decision making and justification. Both types of problems appear with different frequency in all the books; each of these types of problems range from 20% of all the problems posed in the chapter in [H4] to 35% in [H6] 1. To solve these two types of problems, the books introduce the following procedures: P1. Fitting the least squares line. Lineal regression by the least squares method is included in all the books, with two different procedures to compute the line of best fit: a) Equation of the line which includes the gravity centre (point whose coordinates are the mean for the two variables); and b) General expression for the lineal function: Regression line Y = f(X) a) y − y = byx ⋅ (x − x) b) y = a + byx ⋅ x Regression line X = f(Y) a) x − x = bxy ⋅ (y − y) b) x = a’ + bxy ⋅ y 1 All the books also introduced the analysis of the sign and strength of correlation between the variables as a previous problem as well as problems related to graphical representation of bivariate data. We are not including here these problems that were analyzed in (Gea et al., 2013). 659 Regression in high school (María M. Gea, Carmen Batanero, Pedro Arteaga, J. Miguel Contreras and Gustavo R. Cañadas) P2. Fitting other regression models. Only [H8] includes the regression towards the median developed by Tukey, which is a robust method in the presence of outliers as well as variable transformation to fit exponential and polynomial models. P3. Computing the determination coefficient and interpreting the goodness of fit. Only a few textbooks introduce the determination coefficient r2 as a measure of the goodness of fit; in all of them the coefficient is previously defined. P4. Prediction with the line of best fit and interpreting the goodness of prediction. All the books propose some tasks where the students have to estimate the response variable Y given a value for the explanatory variable X. Some of them ([H1], [H2]) suggest that when the correlation coefficient is strong it is possible to use the same line to predict X from a value of Y. Since this property is not general (as there are two different regression lines) students could make an incorrect generalization. Some books qualitatively evaluate the goodness of fit by comparing the estimated and observed values for isolated data. This informal method is not reliable, as the reliability of estimation depends on the goodness of fit (given by the determination coefficient) as well as on the closeness of the estimated value to the centre of the distribution of the explanatory variable. In Table 1 we observe that all the books include the least square line, as well as its use for prediction, while only one includes the Tukey line and a few the evaluation of the goodness of fit. A missing point is the use of informal “eye fitting” methods that are useful to build the students’ intuitions and can be implemented with applets (e.g., //docentes.educacion.navarra.es/ msadaall/geogebra/figuras/e3regresion.htm). Results are better than those by Lavalle, Micheli and Rubio (2006); where only half the books include the line of best fit and 60% of the books include prediction activities. CONCEPTS AND PROPERTIES In our analysis we found the following concepts linked to regression: C1. Dependent (response) and independent (explanatory) variable: While correlation is symmetric, regression is asymmetric; for this reason we should discriminate the response and explanatory variables (Estepa, 1994). Only a few books make the distinction explicit although in other books it is implicit when they introduce two different regression lines. C2. Model of fit. The idea that the regression line is only a model (and therefore does not exactly coincide with all the data) is only implicitly introduced; a couple of books make this idea explicit: “When there is strong correlation between X and Y the analysis of regression helps to find a mathematical function as a model to fit the data. This function can be a straight line, a parabola, exponential...“ ([H4], p.226). C3. Line of best fit (linear model). All the textbooks include the definition and explanation of the minimum squares method (in an informal way); however, only a few of them justify the utility of the model to estimate values of Y in situations where the variable is difficult to measure. Moreover, as in Sánchez Cobo (1999), few texts highlight the predictive utility of the regression line. C4. Regression coefficients. Since there are two possible lines of best fit (depending on which is the explanatory and response variable) there are two different regression coefficients, but only a few books make this explicit. They also include the interpretation of these coefficients: “The line that minimizes the sum of residuals Σd2i is given by the following expression: y = y + σxy (x - x)/σx2. The slope σxy/σx2, is the regression coefficient” ([H1], p. 230). Procedures H1 H2 H3 H4 H5 H6 H7 H8 P1. Fitting the least squares line x x x x x x x X P2. Fitting other regression models X P3.Computing the determination coefficient and interpreting the goodness of fit P4. Prediction and interpretation (line of best fit) x x x x x x x x x X Table 1: Regression analysis procedures included in the books 660 Regression in high school (María M. Gea, Carmen Batanero, Pedro Arteaga, J. Miguel Contreras and Gustavo R. Cañadas) C5. Goodness of fit. Coefficient of determination. These concepts help the student understand the meaning of regression; however they are only included in [H5] and [H6], where the accuracy of the model is identified with the accuracy in the prediction for any particular point. This is not true, as the accuracy is higher when the point approaches the centre of the distribution of the explanatory variable. C6. Non lineal models of fit. Regression is a general method for understanding relationships between variables (Moore, 2005), and therefore it is necessary to introduce different models of fit; however, only a few textbooks implicitly define non lineal models, where [H5] and [H6] define them explicitly. from the points to the line minimum. Usually the explanation is only visual (a formal deductive proof is avoided). P2. Two different regression lines. Most of the books implicitly remark that there are two different regression lines and part of them warns the students of the danger of using an inadequate line to make a prediction. Two books ([H2] y [H8]) do not remark on this property. This omission may reinforce the deterministic conception of some students (Estepa, 1994), since in deterministic dependence there is only one algebraic expression (function) to express the dependence. P3. Percentage of variance explained (r2). The determination coefficient measures the goodness of fit. Some We summarize the definition of concepts in Table 2, textbooks also analyze its interpretation as the perwhere we observe the predominance of adding ex- centage of variance explained by the regression line: amples to the definitions; usually the texts include “(r2x100)% is the percentage of variance of Y explained scatter plots with the line of best fit added to show the by the value of X”([H6], p.185). residuals. The line of best fit is introduced in all the books and is generally defined both in a structural P4. Estimation using the regression line. The regression and operational way (Sfard, 1991). The presentation line serves to predict the value of response (Y) given is very similar to that in Sánchez Cobo (1999). Other a value of the explanatory variable (X). The books definitions (regression coefficients, dependent and implicitly indicate that these estimates are only apindependent variable, goodness of fit and non linear proximations. They insist that, contrary to functional models) are missing in some textbooks or are only dependence, there are several values of Y for a given defined in an operational way. The definitions are value of X, and the regression line provides the averintroduced in different orders; sometimes the exam- age of all these values: “These estimates are approximaples are followed by the definition and vice-versa. In tions and involve a probability; it is probable that when the same way the order to introduce operational or x = x0 the value of y is approximately y(x0). ([H1], p.230). structural definitions also varies. P5. The regression line crosses the distribution centre of Properties gravity, a property only included in half the books in The textbooks add different properties to the defini- the study by Sánchez Cobo (1999). tion of the concepts or to relate different concepts, as described below: P6. Estimates are more accurate for values closer to the centre of gravity. However some books only judge the P1. Least squares property. Most textbooks explain reliability of estimates by the value of the correlation that the regression lines make the sum of residuals coefficient. Concepts H1 H2 H3 H4 C1. Dependent and independent variable H5 H6 H7 O C2. Model of fit C3. Line of best fit (linear models) ESO EOS C4. Regression coefficients O SO ES SE SO SOE SOE H8 O SOE SOE SO O C5. Goodness of fit. SOE OE C6.Non linear models SOE SOE E = Examples; O = Operational definition; S =Structural definition Table 2: Concepts linked to regression and type of definition 661 Regression in high school (María M. Gea, Carmen Batanero, Pedro Arteaga, J. Miguel Contreras and Gustavo R. Cañadas) P7. Reliability of estimates and sample size. It is included only in a few books: “The estimate accuracy increases with the number of data; the regression line computed with few data has little reliability even if r is high” ([H5], p. 260). P8. Strength of correlation and angle of regression lines: This angle varies from perpendicular lines (independence) to only one line (perfect linear dependence). P9. Regression line, covariance, correlation. Covariance and correlation are interpreted as regards the closeness of the points to the regression line. Their sign is related to the slope in the regression line: “Depending on the position of (xi, yi) as regards (x, y), the product (xi − x) ⋅ (yi − y) will be positive or negative. If many points are close to a line with a positive slope, most of these products are positive and the covariance and correlation are positive” ([H1], p. 228). These properties were not found in the books analyzed by Sánchez Cobo (1999). P10. Product of regression coefficients. Some books suggest that the product of regression coefficients is equal to the square of the correlation coefficient: r2. This property was found in most books in Sánchez Cobo’s research (1999) but is found in only two books in our study. PP11. Correlation and reliability of estimates. Most textbooks relate both concepts: “The higher the correlation coefficient r, the higher the reliability of estimates: when r is close to zero, there is not much sense in doing an estimation; as r approaches to 1 or -1, the real values will approach our estimates; when r = 1 or r = -1, real values and estimates coincide” ([H4], p. 226). In Table 3 we summarize the properties of regression included in the books. There are great differences between textbooks, because while some of them ([H4], [H8]) hardly describe any of the properties analyzed, others ([H3]) include almost all of them. The most frequent property is the least squares, the existence of two different regression lines, estimation with the line, centre of gravity crossing the line, and relationship of reliability in the estimate, centre of gravity and correlation coefficient. Globally the books introduce a rich set of properties of regression. We remark that some textbooks do not include the properties P8 and P9; this omission may reinforce the students’ failures in interpreting the relationship between the correlation coefficient and the slope of the regression line described by Sánchez Cobo (1999). DISCUSSION AND IMPLICATIONS FOR TEACHING Our results suggest little changes in the presentation of regression in the high school textbooks, as regards the analysis by Sánchez Cobo (1999), although the books studied by this author were published between 1977 and 1990. Our analysis complement that research and that by Lavalle, Micheli, & Rubio (2006), because neither of these previous studies analyzed the properties linked to regression, in spite of the fact that Sfard (1991) considered that the properties are an essential part of the concepts. This study also complements our previous studies: (Gea et al., 2013), where we analyzed with more detail the problem-situations used to contextualize correlation and regression, and (Gea et al., 2014) where we described the symbolic, verbal and graphical language used in this theme. H1 H2 H3 P1. Least squares property x x x P2.Two different regression lines x x H4 x P3. Percentage of variance explained (r2) H6 H7 H8 x x x x x x x x x x x x P4. Estimation using the regression line x x x P5. Regression line and centre of gravity x x x x x x x P6. Reliability of estimates and centre of gravity x x x x x x x x x x x P7. Reliability of estimates and sample size P8. Strength of correlation- angle of regression lines x x P9. Regression line, correlation, covariance x x P10. Product of regression coefficients x x P11. Correlation coefficient – reliability of prediction x x x x H5 x x x x x x x Table 3: Properties of regression 662 Regression in high school (María M. Gea, Carmen Batanero, Pedro Arteaga, J. Miguel Contreras and Gustavo R. Cañadas) Although all the textbooks introduce linear regression and propose methods to compute the line of best fit and make predictions with the same, only a few of them introduce procedures to compute the determination coefficient and its interpretation as a percentage of explained variance; however these properties are introduced theoretically with no practical applications or procedures related to the same. Similar to the study of Lavalle, Micheli, & Rubio (2006), only a few textbooks introduce examples of non linear regression, even when a few propose tasks where different models would be preferable. This coincides with Sánchez Cobo’s research (1999) where some textbooks introduced examples of non linear regression without discussing these models. We found few definitions of the concepts linked to regression, apart from the definition of the line of best fit. This fact could be explained because these textbooks use most of the available space for this theme in the study of correlation (computing correlation coefficients and interpreting its sign and strength), as we are shown in (Gea et al., 2013). Although correlation is no doubt an important concept, it doesn´t make much sense that the books devote so much space to its study if this study is not completed with the study of regression; the need to fit a model to the data is the reason to study correlation between variables; the isolated study of correlation is useless. We therefore recommend reinforcing the study of other concepts linked to regression, such as model, centre of gravity, regression coefficients, and parameters of the line of best fit (slope; coordinate at the origin). to study the reliability of prediction, with no connection, for example of the sign of correlation and the slope of the regression line. In the same way there is no discussion of examples where a low correlation coefficient may be associated to a strong non linear dependence; for example a parabola. It is also important to emphasize explicitly the existence of two different regression lines; that may sometimes be very close when r is close to +1 or -1, but may be very different in the general case. Many times the difference is only implicit. When comparing the different books, [H5] and [H6] are far more complete than the other books, as they present all the procedures for the linear model; they are the only books that define the goodness and non linear models (although they do not include the procedures for these models) and introduce the majority of properties analyzed. ACKNOWLEDGEMENT Project EDU2013-41141-P (MEC) and group FQM126 (Junta de Andalucía). REFERENCES Anthony, G., & Walshaw, M. (2009). Effective pedagogy in mathematics. Geneva: International Bureau of Education. Chapman, L. J., & Chapman, J. P. (1967). Genesis of popular but erroneous psychodiagnostic observations. Journal of Abnormal Psychology, 72(3), 193–204. Engel, J., & Sedlmeier, P. (2011). Correlation and regression in the training of teachers. In C. Batanero, G. Burrill, & C. 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