[R] R Error: System is computationally singular

[David Winsemius]

On May 24, 2012, at 1:57 PM, Nathan Svoboda wrote:

> Greetings,
>
> I am trying to fit a zero-inflated Poisson model using zeroinfl()  
> from the
> pscl library. I have 5 covariates (4 continuous, 1 categorical); the
> categorical variable has 7 levels.  I have had success fitting  
> models that
> contain only the continuous covariates; however, when I add the  
> categorical
> variable to any of the models (or if I run it by itself) I get the  
> following
> error:
>
> Error in solve.default(as.matrix(fit$hessian)) :
>
>  system is computationally singular: reciprocal condition number =
> 3.46934e-20
>
> The code I am using is:
>
> library(pscl)
> f1 <- formula(LOCS ~ as.factor(LCOVER) + D_ROADS + D_WATER + D_EDGE +
> D_GRASS)
> ZIP1 <- zeroinfl(f1, dist="poisson", link = "logit", data = FAWNS)
>
> There is no correlation between my covariates. Also, I tried  
> reducing my
> categorical covariate to 3 levels and still receive the same error.  
> Can
> anyone suggest why I may be getting this error when I add the  
> categorical
> covariate?
>

What does this show:

with( FAWNS, table(LOCS, LCOVER) )

-- 
David Winsemius, MD
West Hartford, CT