Hello All,

This is my first time using this platform, so apologies in advance for any faux pas. Also, I am new to model building and statistics world, so please bear with me.
I am using a ZTNB model where my outcome is the number of waivered physicians at the ZCTA (Zip-code tabulation area) level and my explanatory variables are community characteristics - median income, race, education, marital status, rurality etc. I already checked for mean=variance assumption which was not met, hence I am using negative binomial and also my outcome has no zeros due to which I am using zero truncated. However, I am completely confused about the exposure variables that needs to be included in the model in order to account for any variance which may lead to biased estimation. Please correct me if I am wrong, but I was thinking to use population size as the exposure because population size will affect the number of waivered physicians available but I am confused (a) if this exposure makes sense and (b) how to incorporate exposure variable in the code. I read about it a lot and I am really confused because most of the forums, it was mentioned to write "exposure(varname)" after the tnbreg command but in one of STATA documentation, I also read to use "vce(cluster varname) nolog" after the tnbreg command. One last method, I read about was to enter "offset (log of variable)". Therefore, it would be really helpful to understand little bit about exposure variables and what is the right way to account for them in ZTNB model. Additionally, it would be nice to know if the way to use exposure variable changes with other count models - poisson, negative binomial, zero-inflated, zero-inflated negative binomial.

I need to submit an abstract for a conference and I am stuck here, so your quick replies can help me a lot.

Thank You,
Sadia Jehan