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I have a data set including (fish) egg volume, Treatments, adult male length, adult female length and the location where parental fish were caught. I first made the mistake and introduced the locations as fixed factors. However, the procedure of eliminating one term of interest after the other worked perfectly fine. When I changed the two location variables to random effects, I got the singular fit error. The model I used is the following:
mod1 <- lmer(egg.volume ~ Treatment + Dam.length + Sire.length + (1|Sire) + (1|Dam) + (1|Location.Sire) + (1|Location.Dam), data = dta, REML = F).
Can anyone explain to me
a) what the error message exactly tells me and
b) how I can solve this issue?