I want to fit a zero-inflated neg. binomial model using
zeronfl(outcol ~ vm + Thursday + Saturday |Saturday + Thursday + vm, data, family="negbin") from the pscl package (this is just a simplied formula). I am receiving an error
Error in solve.default(as.matrix(fit_count$hessian)) : Lapack routine dgesv: system is exactly singular: U[2,2] = 0
By searching the internet I found that Achim Zeileis, the maintainer of the package told other people that the vaiance in the outcome for some covariates may be too small. Now I know that my outcome is always zero for
Saturday==1, which is one reason I have so many zeros in the outcome and hence want to use a zeroinflated model.
When I do not use Saturday as covariate then I do not get the error. So is this the problem and, if yes, how can I fix it? And why is this even a problem? I used a common Poisson model as well and it doesn't mind the zeros in the outcome for
Saturday==1. I understand that I probably should not use Saturday for the count model, but for the zero model it makes sense to it I guess?