st: Two part model - How to combine models?

[Tamara Pejovic <tamara.pejovic@ic.ac.uk>]

Hi,
I have a quick question. I have a response variables that is positively 
skewed and contain a substantial proportion of zeroes. Since a common 
method for analyzing this type of data is a two-part model I have the 
analysis of three stages:
The first involved creating two sets of data from the original: one 
showing whether or not the problem is present and the other indicating 
the "level of the problem" when problem is present.
The second stage involved modelling occurrence of problem, using 
logistic regression, and separately modeling the level data using 
ordinary regression.

Finally, the third stage should be combining the two models in order to 
estimate the expected "level of the problem" for a specific set of 
values of possible predictors.

My question is how to do this? Is it just enough to multiple 
probabilities using conditional probabilities rule? Does STATA have a 
module for solving two-part models?

Thanks,
Tash


*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/