st: likelihood ratio test error message

["Rivera, Semilla M." <srivera@sph.uth.tmc.edu>]

Hello,
 
I am attempting to run a lrtest in STATA and get the following error message.  What can I do?
 
Thank you.
Semilla Rivera

. lrtest, using (1)
 
observations differ: 2098 vs. 2216
r(498);
 

*
*   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/