[R-sig-eco] Statistical question regarding interaction terms (Cortney Watt)
Philip Dixon
pdixon at iastate.edu
Thu May 7 12:37:05 CEST 2009
Courtney,
There are at least two possible explanations:
1) Tests of interactions are less powerful than tests of simple effects (i.e.
tests of canopy cover in each zone). This is easy to see by defining
contrasts among cell means in the 2 x 2 table of means (zone * canopy) and
calculating the se of a simple effect and the se of a difference in simple
effects (i.e. the interaction).
2) The denominator variances, site(I*C), are not similar in the three zones.
The test of interaction in the overall anova is based on a pooled site(I*C)
variance. The analysis of each separate zone estimates three site(I*C)
variances. If the site(I*C) variance is large at your third elevation and
smaller at the other two elevation, the tests of canopy effects can give
different p-values, even if the canopy effects (differences in means) are
similar in the three zones.
Best wishes,
Philip Dixon
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