No residual variance using MCMCglmm
Dear all, I have recently bumped twice in the same issue running glmm in MCMCglmm: the posterior distribution of residual collapses on 0. While I have often seen it for other effects (e.g ID) and interpreted it as evidence of non existence / non significance of these effects, I can not get why residual variance would not be well defined. More specifically, with priors V=1, nu=0.02, I was trying to estimate additive genetic variance in age at first breeding. I first tried a Poisson distribution and the posterior distribution of the residual looked more or less ok, although not perfectly bell shaped. Then I thought as age at first breeding could not be zero, that a zero truncated Poisson might be better but then the posterior distribution of residual variance totally collapses on zero. As I thought it could be due to over parametrisation, I rerun the model with only intercept but results were the same. Is it a problem with the variables distributions not really fitting the distribution I'm specifying? Any help would be greatly appreciated! Many thanks in advance Celine
Celine Teplitsky UMR 7204 - CESCO D?partement Ecologie et Gestion de la Biodiversit? CP 51 55 rue Buffon 75005 Paris Webpage : http://www2.mnhn.fr/cersp/spip.php?rubrique96 Fax : (33-1)-4079-3835 Phone: (33-1)-4079-3443