Interpreting tobit coefficients of 0

Question

I'm using a tobit model for a left censored dataset in R, and including both continuous and categorical predictor variables. I've converted the factors to indicator coding for each level. I was initially getting errors when trying to run a summary of the model output, which I discovered was due to the fact that some levels of the categorical variables have a coefficient of 0, even though there are observations for those levels. I'm setting a reference level for the category using the level with the most observations.

I can't seem to find anything about why this would be happening. This isn't survival regression where 0 coefficient is a soft model selection, is it?

If it is a form of model selection, how do I handle that given the factor itself? I know in ridge regression I could use a form of grouping to test the variable overall. Could I use a similar approach here?

Answer 1

The betas at zero were appearing only when using the {AER} Tobit function. Using the {VGAM} package with the vglm function returned values for the previously missing coefficients. I'm assuming this is a problem with some levels of the categorical variables only having a few observations. Combining those levels with others when creating the indicator variables fixed all problems with the models.