Unit 7: ANCOVA

For this assignment, you should complete two analyses and submit them in one file. The first should be a one-way ANOVA using GPA as the dependent variable and repeat as the independent variable. The second analysis should be an ANCOVA using the same independent and dependent variables and using IQ as a covariate. The data file for this unit is a subset of 88 cases from the data file that was used for the Unit 3 assignment (the Howell and Huessy data file). See the unit 7 datafile for all the data needed for this activity. The GPA variable represents the grade point average of each student in ninth grade. The variable repeat is a dichotomous variable that indicates whether a student repeated the grade level the year before. IQ scores were obtained from a group IQ test.

SPSS Hints

Complete the ANCOVA in SPSS by selecting Analyze / General Linear Model / Univariate . This is the same procedure that you used to complete the factorial ANOVA for Unit 5. In this assignment, there will be one fixed independent variable (repeat), one dependent variable (GPA) and one covariate (age). Use the same procedure to compute the ANOVA with repeat and GPA first, then do the ANCOVA by adding IQ as the covariate. This should make comparing the results more straightforward. Also, remember to do the usual variable screening and description, and compute the Pearson r between IQ and GPA to determine if IQ is a good candidate for being a covariate.

The general linear model (GLM) uses regression to compute ANOVA and ANCOVA. If GPA was the criterion variable in regression, repeat was the predictor, and IQ was also a predictor that was entered into the regression prior to repeat, then one could determine if repeat accounted for a significant portion of the variability in GPA after the effect of IQ had been removed form GPA. This is exactly what happens when you use GLM to do ANCOVA.

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