I'm modeling the number of doctor visits (a count variable) on factors such as income, chronic condition, insurance, etc. I use the canned Stata command
poisson y x1 x2, etc.
I have the following two questions:
1) How come Stata does not allow me to execute
predict resids, residuals after I've ran the above regression? I then tried to generate the residuals using the formula "y-yhat", yet I got a residual with a negative average mean (-6.96e-09...) - any thoughts here?
2) Should I be using robust SE (i.e.
vce(robust)) here? I do think there can be heteroskedasticity issues since the number of doctor visits may vary highly among different values of the income variable; however, I'm not sure if robust SEs can be applied as freely in the case of nonlinear models such as Poisson.