Lynn Govaert
2014-Aug-21 09:12 UTC
[R] Calculating contrasts in Anova with interaction term
Hi all, I have the following variables: a trait z, Treatment A and B, Population 1 and 2 (each having undergo the treatment), and their interaction. Now, I'm doing an Anova as follows aov( z ~ Treatment * Population ) I now want to compare the values of population 1 which underwent treatment A with the values of population 2 which underwent treatment B using contrasts. I found a very useful explanation for calculating contrasts on the internet ( http://r-eco-evo.blogspot.be/2007/10/one-of-most-neglected-topics-in_06.html), however, they did not explain it for an interaction term. Now I thought, if I just make a new variable for my interaction term, which is a categorical variable having 4 levels (level 1: pop 1 with treatment A, level 2: pop 1 with treatment B, level 3: pop 2 treatment A and level 4: pop 2 treatment B). If I then construct my contrast vector as con <- c(1,0,0,-1) Then do contrasts(data$trait) <- con Then my model model <- aov( z ~ Treatment + Population + InteractionTerm ) with the InteractionTerm the new created variable and then do what they did summary(model, split = list(trait = list("comparison"=1))) I get the following result Treatment 1 0.00246 0.00246 3.059 0.0816 . Population 1 0.05643 0.05643 70.241 4.75e-15 *** IntTerm 1 0.02869 0.02869 35.713 8.37e-09 *** IntTerm: comparison 1 0.02869 0.02869 35.713 8.37e-09 *** Residuals 236 0.18959 0.00080 It is just so strange that the two last rows are exactly the same. So I guess I'm doing something wrong. As a second related question. I also found another method to calculate contrasts. Using the Phia package in R. It works fine, but I don't find how I can make a link between my two variables, so it takes pop 1 with treatment A and pop 2 with treatment B to compare those two. I only find how I can make combination within my treatment variable or within my population variable. And that is not what I want. You can find the information on http://cran.r-project.org/web/packages/phia/vignettes/phia.pdf If anyone knows what I'm doing wrong in the first example or if it makes sence that they could be the same. Or an answer to my second question. I would be very happy. Thanks in advance! Lynn [[alternative HTML version deleted]]