Asking, are simple effects different from 0
On 3/5/2008 1:32 PM, jebyrnes wrote:
Ah. I see. So, if I want to test to see whether each simple effect is different from 0, I would do something like the following: cm2 <- rbind( "A:L" = c(1, 0, 0, 0, 0, 0), "A:M" = c(1, 1, 0, 0, 0, 0), "A:H" = c(1, 0, 1, 0, 0, 0), "B:L" = c(1, 0, 0, 1, 0, 0), "B:M" = c(1, 1, 0, 1, 1, 0), "B:H" = c(1, 0, 1, 1, 0, 1))
That does not corresponds to what I think of as the simple effects. That specifies the six cell means, but it does not *compare* any cell means. I think of a simple effect as the effect of one factor at a specific level of some other factor.
summary(glht(fm, linfct = cm2), test = adjusted(type="none")) Correct? What is the df on those t-tests then? Is it 48?
Yes, df = 48 for each contrast.
Interestingly, I find this produces results no different than fm2<-lm(breaks ~ tension:wool+0, data=warpbreaks) summary(fm2)
Yes, but those are not what I would call the simple effects. Those are essentially one-sample t-tests for each of the 6 cell means.
Also, here, it would seem each t-test was done with the full 48df. Hrm.
The df are based on the whole model, not the 9 observations in one cell.
Chuck Cleland wrote:
Each column corresponds to one of the coefficients in the model, and each row specifies a particular contrast. The numbers in the matrix indicate how the model coefficients are combined to indicate a particular difference in means. For example, the first row indicates that the third coefficient (woolB) is multiplied by -1. The baseline categories are A and L for the wool and tension factors, so the woolB effect in fm is the simple effect of B vs. A in the baseline category of the tension factor. Multiplying this coefficient by -1 produces an A vs. B comparison in the baseline category of the tension factor.
Chuck Cleland, Ph.D. NDRI, Inc. 71 West 23rd Street, 8th floor New York, NY 10010 tel: (212) 845-4495 (Tu, Th) tel: (732) 512-0171 (M, W, F) fax: (917) 438-0894