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re: st: covariance structure name?


From   David Airey <david.airey@vanderbilt.edu>
To   statalist@hsphsun2.harvard.edu
Subject   re: st: covariance structure name?
Date   Fri, 18 Aug 2006 22:50:44 -0500

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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