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Interactions in MRM
2 messages · Leamy, Larry, Sarah Goslee
Hi Larry,
On Thu, Nov 14, 2013 at 9:31 AM, Leamy, Larry <ljleamy at uncc.edu> wrote:
Dear Dr. Goslee: I found some posts about potential interactions in MRM, and I wonder if it is acceptable to construct an interaction term as follows? leaf1 = MRM(gendist ~ regiondist + envirdist +I(regiondist*envirdist) + moldist, mrank = TRUE, nperm=1000) (I am using mrank = TRUE, incidentally, because our gendist matrix has a number of 0 values)
Nothing has changed since those earlier discussions: MRM will not process interaction terms. You can use standard regression methods to get coefficients, but not measures of significance because os the permutation test needed for properly testing hypotheses based on dissimilarities.
If this is not acceptable, what if I simply multiply the elements in the two matrices to generate a separate (interaction) matrix? This is the usual way I have formed interactions in multiple regression models in SAS.
I don't know of any testing of the effectiveness of that method, and cannot recommend it. The extensive literature on partial Mantel tests should give you some idea that dealing with anything involving dissimilarities is not statistically straightforward. That said, if you find some research on the subject or develop your own rigorous testing I will add your findings into the ecodist package. :)
Which (if either) is preferable, and is there any problem with significance testing with an interaction term in the model? Thanks much for your help. Best, Larry Leamy
Sarah Goslee http://www.functionaldiversity.org