Dear Statalist,
I would like to use GEE command to analysis a two-within factors experiment.
In this experiment the same 16 patients (id) received consecutly
3 different treatments (treat) and their heart rate (hr) was measured every
minute during 15 minutes (time). t0 represented the heart rate. Then I run
the following command line :
xi: xtgee fc t0 i.treat*time, i(id) t(time) robust and I get
results.
However I tried to apply autoregressive correlation structure to the data :
xi: xtgee fc t0 i.treat*time, i(id) t(time) robust corr(ar1) and I
get this error message:
"note : observations not equally spaced
"modal spacing is delta time=0"
"16 groups omitted from estimation"
What"s wrong?
Sincerely yours,
Xavier Viviand, M.D.
*
* 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/
