ICOTS6, 2002: Hettmansperger and Elmore 1 TESTS FOR INTERACTION IN A TWO-WAY LAYOUT: SHOULD THEY BE INCLUDED IN A NONPARAMETRICS COURSE?

We first note that tests for interaction are missing in virtually all textbooks on nonparametric statistics. We will discuss some reasons why this is so. We then make a case for featuring tests for interaction in the course. By learning how to use median polish and graphical displays students can begin to conceptualize what an interaction means. This will strengthen their understanding of additive models as well. After a conceptual basis for understanding interaction is in place, we can then proceed to design tests for interaction. They will not be strictly nonparametric. This provides a good opportunity for discussion of what it means to have a nonparametric test and why it is impossible to construct an ordinary permutation test for interaction.