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re: st: covariance structure name?
Please note that this is a stats question and not a Stata question;
which is the purpose of the list. If you have a stats question try
MedStats.
It would seem you have a autoregressive structure. The autoregressive
correlation model formalizes the idea that the magnitude of
correlation among observations "decays" or "drop off" as they become
farther apart.
Raphael
No, I did not frame the question with regard to a specific Stata
command...but your answer did help in that regard.
The six measures were from consecutive minutes over a six minute
interval. Mouse behavior was measured. Multiple mice are each tested
under a saline and two drugs. Replicate mice are nested in 20
isogenic lines, where lines are nested in two genotype classes. The
effect of genotype on drug response was of interest. I used a sum of
the behavior measure across all six minutes, and ran the following
model:
xi: xtmixed sum_6_min i.drug*i.genotype || strain: || mouse:
This is fine since I was not really interested in the time aspect of
the 6 measures, but I was thinking about running the model on the raw
data. My question was really about the setting of the cov() option in
xtmixed should I decide to try this.
I thought autoregressive had the same variances along the diagonal.
In any case, I think it is moot since I only see unstructured as an
alternative in xtmixed.
Where is MedStats? I could not find a google link that seemed to make
sense. I would like a good statistics listserv for questions about
statistics generally.
Thanks.
-Dave
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