xtmelogit: negative variance?

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xtmelogit: negative variance?

Hanne F
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Hi,

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,
Hanne
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Re: xtmelogit: negative variance?

clover-21
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Hi Hanne,

I also encounter the same problem when running xtmelogit. My dependent variable is binary and includes a large proportion of zeros (about 90%). I'm not sure whether this could be the cause. Have you figured it out yet?

Thanks and warmest regards,
Coco

Hanne F wrote
Hi,

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,
Hanne