Karl, The definition and interpretation of contrasts is part of any intermediate design of experiments text.
Contrasts for interactions say that the effect of moving "from level 1 of A to level 2 of A" depends on the level of B. I will use notation YAB to indicate the levels of A and B. For example, if (Y11 - Y21) differs from (Y12 - Y22) we say that A and B have an interaction. When A and B interact, then the interpretation of "main effects" is ambiguous at best. Instead, we use the concept of simple effects, which, for example, are the effects of changing levels of A while holding the levels of B constant. Interpreting the interactions themselves depends on knowing something about the structure of the design, for example whether the effects (A and B, here) are treatments, blocks, nested effects, or repeated measures. Simple effects are usually interpretable. Interactions are tougher. Rich <https://mail.google.com/a/temple.edu/?AuthEventSource=SSO#inbox> [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
