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Analysis of Variance |
F tests are formed from this table in the same fashion that was explained previously in the section "Analysis of Variance." In this case, there are three null hypotheses being tested: class means are all the same for the main effect DRUG, the main effect DISEASE, and the interaction effect DRUG*DISEASE. Begin by examining the test for the interaction effect since a strong interaction makes the interpretation of main effects difficult if not impossible. The computed F statistic is 1.7406 with a p-value of 0.1271. This gives little evidence for an interaction effect. Now examine the main effects. The computed F statistic for DRUG is 15.8053 with a p-value less than or equal to 0.0001. The computed F statistic for DISEASE is 4.2220 with a p-value of 0.0193. While both effects are significant, the DRUG effect appears to be stronger. Now you have more information about which means are significantly different. The results of the F test in the Analysis of Variance table indicated only that at least one of the class means is different from the others. Now you know that the difference in means can be associated with the different levels of the main effects, DRUG and DISEASE.
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