Generally, a priori comparisons differ from a posteriori comparisons because O the only correct way to calculate a priori comparisons is to perform them using SPSS. O researchers choose to perform a priori comparisons when they want to test all possible simple comparisons. certain a priori comparisons are selected by the researcher based on predictions informed by the results of previous studies. choosing a priori comparisons means you will have no greater chance of Type I error than the probability of Type I error associated with the overall analysis of variance