Dear all, I am performing a glimmix model on count data (brood size: number of nestlings in a nest). The distribution ranges from 1 to 6 nestlings per nest. I am trying to test a fixed factor (treatment) with two levels. The models also includes a random term (the brood identity where the parents were born). The model shows underdispersion as the generic chi-square /DF shows a 0.29 value, and the distribution shows a large peak at the central value of 3 chicks. First, I am unsure about the necessity of correcting for underdispersion here. Apparently, there is a debate on this as by correcting it the possibility of inflating type II errors increases. I would want to ask you about your opinion on this question. Second, I do not know how to fix it, how to correct underdispersion. Please, see the model below. What syntaxis should be added? Finally, I would also know if some correction for the degrees of freedom must be done. I have tried Satt but it shows some problems as the DF changes to 1 (some problem with the random term, which is never significant). proc glimmix ; class idbrood treatment ; model broodsize= treatment/dist=poisson link=log ; random idbrood/group=treatment; covtest/wald; run; I would be very grateful for your help and comments. Thanks in advance, Carlos
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