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CHAPTER 4 Exploratory Factor Analysis and Principal Components

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CHAPTER 4 Exploratory Factor Analysis and Principal Components CHAPTER 4 Exploratory Factor Analysis and Principal Components Analysis Exploratory factor analysis (EFA) and principal components analysis (PCA) both are methods that are used to help investigators represent a large number of relationships among normally distributed or scale variables in a simpler (more parsimonious) way. Both of these approaches determine which, of a fairly large set of items, “hang together” as groups or are answered most similarly by the participants. EFA also can help assess the level of construct (factorial) validity in a dataset regarding a measure purported to measure certain constructs. A related approach, confirmatory factor analysis, in which one tests very specific models of how variables are related to underlying constructs (conceptual variables), requires additional software and is beyond the scope of this book so it will not be discussed. The primary difference, conceptually, between exploratory factor analysis and principal components analysis is that in EFA one postulates that there is a smaller set of unobserved (latent) variables or constructs underlying the variables actually observed or measured (this is commonly done to assess validity), whereas in PCA one is simply trying to mathematically derive a relatively small number of variables to use to convey as much of the information in the observed/measured variables as possible. In other words, EFA is directed at understanding the relations among variables by understanding the constructs that underlie them, whereas PCA is simply directed toward enabling one to derive fewer variables to provide the same information that one would obtain from the larger set of variables. There are actually a number of different ways of computing factors for factor analysis; in this chapter, we will use only one of these methods, principal axis factor analysis (PA). We selected this approach because it is highly similar mathematically to PCA. The primary difference, computationally, between PCA and PA is that in the former the analysis typically is performed on an ordinary correlation matrix, complete with the correlations of each item or variable with itself. In contrast, in PA factor analysis, the correlation matrix is modified such that the correlations of each item with itself are replaced with a “communality”—a measure of that item’s relation to all other items (usually a squared multiple correlation). Thus, with PCA the researcher is trying to reproduce all information (variance and covariance) associated with the set of variables, whereas PA factor analysis is directed at understanding only the covariation among variables. Conditions for Exploratory Factor Analysis and Principal Components Analysis There are two main conditions necessary for factor analysis and principal components analysis. The first is that there need to be relationships among the variables. Further, the larger the sample size, especially in relation to the number of variables, the more reliable the resulting factors. Sample size is less crucial for factor analysis to the extent that the communalities of items with the other items are high, or at least relatively high and variable. Ordinary principal axis factor analysis should never be done if the number of items/variables is greater than the number of participants. Assumptions for Exploratory Factor Analysis and Principal Components Analysis The methods of extracting factors and components that are used in this book do not make strong distributional assumptions; normality is important only to the extent that skewness or outliers affect the observed correlations or if significance tests are performed (which is rare for EFA and PCA). The normality of the distribution can be checked by computing the skewness value of each variable. Maximum likelihood estimation, which we will not cover, does require multivariate normality; the variables need to be normally distributed and the joint distribution of all the variables should be normal. Because both principal axis factor analysis and principal components analysis are based on correlations, independent sampling is required and the variables should be related to each other (in pairs) in a linear 68 EXPLORATORY FACTOR ANALYSIS AND PRINCIPAL COMPONENTS ANALYSIS 69 fashion. The assumption of linearity can be assessed with matrix scatterplots, as shown in Chapter 2. Finally, each of the variables should be correlated at a moderate level with some of the other variables. Factor analysis and principal components analysis seek to explain or reproduce the correlation matrix, which would not be a sensible thing to do if the correlations all hover around zero. Bartlett’s test of sphericity addresses this assumption. However, if correlations are too high, this may cause problems with obtaining a mathematical solution to the factor analysis. • Retrieve your data file: hsbdataNew.sav. Problem 4.1: Factor Analysis on Math Attitude Variables In Problem 4.1, we perform a principal axis factor analysis on the math attitude variables. Factor analysis is more appropriate than PCA when one has the belief that there are latent variables underlying the variables or items measured. In this example, we have beliefs about the constructs underlying the math attitude questions; we believe that there are three constructs: motivation, competence, and pleasure. Now, we want to see if the items that were written to index each of these constructs actually do “hang together”; that is, we wish to determine empirically whether participants’ responses to the motivation questions are more similar to each other than to their responses to the competence items, and so on. Conducting factor analysis can assist us in validating the data: if the data do fit into the three constructs that we believe exist, then this gives us support for the construct validity of the math attitude measure in this sample. The analysis is considered exploratory factor analysis even though we have some ideas about the structure of the data because our hypotheses regarding the model are not very specific; we do not have specific predictions about the size of the relation of each observed variable to each latent variable, etc. Moreover, we “allow” the factor analysis to find factors that best fit the data, even if this deviates from our original predictions. 4.1 Are there three constructs (motivation, competence, and pleasure) underlying the math attitude questions? To answer this question, we will conduct a factor analysis using the principal axis factoring method and specify the number of factors to be three (because our conceptualization is that there are three math attitude scales or factors: motivation, competence, and pleasure). • Analyze → Dimension Reduction → Factor… to get Fig. 4.1. • Next, select the variables item01 through item14. Do not include item04r or any of the other reversed items because we are including the unreversed versions of those same items. Fig. 4.1. Factor analysis. 70 CHAPTER 4 • Now click on Descriptives… to produce Fig. 4.2. • Then click on the following: Initial solution and Univariate Descriptives (under Statistics), Coefficients, Determinant, and KMO and Bartlett’s test of sphericity (under Correlation Matrix). • Click on Continue to return to Fig. 4.1. Fig. 4.2. Factor analysis: Descriptives. • Next, click on Extraction… This will give you Fig. 4.3. • Select Principal axis factoring from the Method pull-down menu. • Unclick Unrotated factor solution (under Display). We will examine this only in Problem 4.2. We also usually would check the Scree plot box. However, again, we will request and interpret the scree plot only in Problem 4.2. • Click on Fixed number of factors under Extract, and type 3 in the box. This setting instructs the computer to extract three math attitude factors. • Click on Continue to return to Fig. 4.1. Fig. 4.3. Extraction method to produce principal axis factoring. • Now click on Rotation… in Fig. 4.1, which will give you Fig. 4.4. EXPLORATORY FACTOR ANALYSIS AND PRINCIPAL COMPONENTS ANALYSIS 71 • Click on Varimax, then make sure Rotated solution is also checked. Varimax rotation creates a solution in which the factors are orthogonal (uncorrelated with one another), which can make results easier to interpret and to replicate with future samples. If you believe that the factors (latent concepts) are correlated, you could choose Direct Oblimin, which will provide an oblique solution allowing the factors to be correlated. • Click on Continue. Fig. 4.4. Factor analysis: Rotation. • Next, click on Options…, which will give you Fig. 4.5. • Click on Sorted by size. • Click on Suppress small coefficients and type .3 (point 3) in the Absolute Value below box (see Fig. 4.5). Suppressing small factor loadings makes the output easier to read. • Click on Continue then OK. Compare Output 4.1 with your output and syntax. Fig. 4.5. Factor analysis: Options. Output 4.1: Factor Analysis for Math Attitude Questions FACTOR /VARIABLES item01 item02 item03 item04 item05 item06 item07 item08 item09 item10 item11 item12 item13 item14 /MISSING LISTWISE /ANALYSIS item01 item02 item03 item04 item05 item06 item07 item08 item09 item10 item11 item12 item13 item14 72 CHAPTER 4 /PRINT UNIVARIATE INITIAL CORRELATION DET KMO EXTRACTION ROTATION /FORMAT SORT BLANK(.3) /CRITERIA FACTORS(3) ITERATE(25) /EXTRACTION PAF /CRITERIA ITERATE(25) /ROTATION VARIMAX /METHOD=CORRELATION. Factor Analysis Interpretation of Output 4.1 The factor analysis program generates a variety of tables depending on which options you have chosen. The first table includes Descriptive Statistics for each variable and the Analyses N, which in this case is 71 because several items have one or more participants missing. It is especially important to check the Analysis N when you have a small sample, scattered missing data, or one variable with lots of missing data. In the latter case, it may be wise to run the analysis without that variable. Indicates how each question is Should be greater than .0001. If very close associated (correlated) with each to zero, collinearity is too high. If zero, no of the other questions. Only part of solution is possible.
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