[R-sig-eco] Statistical question regarding interaction terms (Cortney Watt)

[Philip Dixon]

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