15.66 Students were classified according to three parental income groups and also according to three possible score ranges on the SAT examination. One student was chosen randomly from each of the nine cross-classifications, and the grade point averages of those sample members at the end of the sophomore year were recorded. The results are shown in the accompanying table.

Income Group Sat Score High Moderate Low Very high 3.7 3.6 3.6 High 3.4 3.5 3.2 Moderate 2.9 2.8 3.0

a. Prepare the analysis of variance table.

b. Test the null hypothesis that the population mean grade point averages are the same for all three income groups.

c. Test the null hypothesis that the population mean grade point averages are the same for all three SAT score groups.