calc_overdispersion <- function(mod) {
## Diagnose overdispersion in a model's response variable
## number of variance parameters in
## an n-by-n variance-covariance matrix
# from http://bbolker.github.io/mixedmodels-misc/glmmFAQ.html#overdispersion
rdf <- df.residual(mod)
rp <- residuals(mod, type = "pearson")
Pearson.chisq <- sum(rp^2)
prat <- Pearson.chisq/rdf
pval <- pchisq(Pearson.chisq, df = rdf, lower.tail = FALSE)
data.frame(chisq = Pearson.chisq, ratio = prat, rdf = rdf, p = pval)
}
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