st: first stage prediction gets negative value for positive observations

[yi jiang <yistata@gmail.com>]

I'm doing a two stage regression. I'm predicting X1 (X1 are all
positive) at the first stage and then use the predicted value of X1 at
the second stage.
However, the predicted value of X1 shows some negative value, which
makes no sense in terms of data--X1 cannot be negative. any thoughts?
*
*   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/