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st: negative variance?

From   Hanne F <>
Subject   st: negative variance?
Date   Tue, 25 Jun 2013 04:11:03 -0700 (PDT)


I am currently using a two-level random slope model, logistic regression.
The reason I use multilevel is to control for a possible design effect. I am
using xtmelogit, but I already tried glamm and xtlogit as well. I have data
for three measurements. When running the same model on the first two
measurements, I do not experience any problems. By contrast, for the third
measurement, I do not seem to get an accurate estimation of the second level
variance. Sd_cons is virtually zero, and the confidence interval is also
odd: I get exactly 0 as a lower point, and a '.' as upper bound. The p-value
indicating whether or not it is necessary to use a multilevel design gives
exactly zero. Can anyone tell me why this is the case?
In a multilevel course based on MLWin I learned that in this case you should
'allow for negative variance'. Can anybody please tell me how I could do
this with Stata?

thank you very much in advance!!

best regards,

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