chapter19.pdf

chapter19.pdf - DISCOVERING STATISTICS USING SPSS Chapter...

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DISCOVERING STATISTICS USING SPSS PROFESSOR ANDY P FIELD 1 Chapter 19: Logistic regression Self-test answers SELF-TEST Rerun this analysis using a stepwise method ( Forward: LR ) entry method of analysis. The main analysis To open the main Logistic Regression dialog box select . Figure 1: Logistic Regression main dialog box In this example, the outcome was whether or not the patient was cured, so we can simply drag Cured from the variable list to the D ependent box (or select it and click on ). There is also a box for specifying the covariates (the predictor variables). It is possible to specify the main effect of a predictor variable (remember, this is the effect on an outcome variable of a variable on its own ). You can also specify an interaction effect, which is the combined effect (on an outcome variable) of two or more variables. To specify a main effect, select one predictor (e.g., Duration ) and then drag it to the C ovariates box (or click on ). To input an interaction, click on more than one variable on the left-hand side of the dialog box (i.e., click on several variables while holding down the Ctrl key, or Cmd on a Mac) and then click on to move them to the C ovariates box. In this example there are only two predictors and therefore there is only one possible interaction (the Duration Intervention interaction), but if you have three predictors then you can select interactions using two predictors, and an interaction involving all
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DISCOVERING STATISTICS USING SPSS PROFESSOR ANDY P FIELD 2 three. In Figure 1, I have selected the two main effects of Duration , Intervention and the Duration Intervention interaction. Select these variables too. Method of regression You can select a particular method of regression by clicking on and then clicking on a method in the resulting drop-down menu. You were asked to do a forward stepwise analysis so select the Forward: LR method of regression. Categorical predictors SPSS needs to know which, if any, predictor variables are categorical. Click on in the Logistic Regression dialog box to activate the dialog box in Figure 2. Notice that the covariates are listed on the left-hand side, and there is a space on the right-hand side in which categorical covariates can be placed. Select any categorical variables you have (in this example we have only one, so click on Intervention ) and drag them to the Cat egorical Covariates box (or click on ). Figure 2: Defining categorical variables in logistic regression Let’s use standard dummy coding ( indicator ) for this example. In our data, I coded ‘cured’ as 1 and ‘not cured’ (our control category) as 0; therefore, select the contrast, then click on and then so that the completed dialog box looks like Figure 2. Obtaining residuals To save residuals click on in the main Logistic Regression dialog box. Select the same options as in Figure 3.
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DISCOVERING STATISTICS USING SPSS PROFESSOR ANDY P FIELD 3 Figure 3: Dialog box for obtaining residuals for logistic regression Further options Finally, click on in the main Logistic Regression
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