Arthur Tabachneck wrote: > Is it legal to test for an interaction without testing for the main > effects of the variables included in those interactions. > (which I recall it isn't), does one still have to account for the > degrees of freedom used by the non-tested main effects?
A proper interaction test compares the improvement to fit achieved by the the interaction *in excess of* the main effects. If there is no improvement, the interaction effect doesn't tell you anything you didn't already know from main effects. So technically, the goodness-of-fit test for interactions includes the consideration of main effects. You have to properly adjust DOF when you're comparing these two models: the interaction one has fewer DOF than the main effects one. I've assumed you're doing this in the context of multiple or logistic regression. There are many views of what an interaction is, and many tests. -- mag. Aleks Jakulin http://ai.fri.uni-lj.si/aleks/ Artificial Intelligence Laboratory, Faculty of Computer and Information Science, University of Ljubljana. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
