st: check if variable is constant within id

["Mak, Timothy" <Timothy.Mak@iop.kcl.ac.uk>]

Dear Statalisters, 

I'm looking for an easy way to check if all the observations for a
particular variable (say var1) within id (a subject identifying
variable) are constant a) for the case where missings can be excluded,
and b) for the case where missings cannot be excluded. The best answer
would be a one-liner, with a short display. I don't want a program to do
this.  

Thanks! 

Tim 

*
*   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/