15.38 Four brands of fertilizer were evaluated. Each brand was applied to each six plots of land containing soils of different types. Percentage increases in corn yields were then measured for the 24 brand-soil-type combinations.

The results obtained are summarized in the accompanying table.

Source of Variation Sum of Squares Between fertilizers 135.6 Between soil types 81.7 Error 111.3

a. Complete the analysis of variance table.

b. Test the null hypothesis that population mean yield increases are the same for the four fertilizers.

c. Test the null hypothesis that population mean yield increases are the same for the six soil types.