Date: Tue, 29 Apr 2003 09:27:31 -0500
Reply-To: Warren Schlechte <Warren.Schlechte@TPWD.STATE.TX.US>
Sender: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From: Warren Schlechte <Warren.Schlechte@TPWD.STATE.TX.US>
Subject: Correcting for Overdispersion in Group without Replication
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Here's an odd question, caused by an odd design. Assume that the unbalanced
problem arises from missing values.
The design has 3 groups of one treatment and 1 group of a comparison
treatment. The outcome is a percentage. So, for example, we could get:
How could one test whether the results in group B are consistent with those
in group A?
As I see it, the most empirical way of approaching this problem would be to
assume that the variance about group A (with replicates) is the same as the
variance about group B (without replicates). However, I am unsure how to
Any help on how I specify that I want to use the variance about group A to
test the null hypothesis that A=B, when the outcome is a percentage would be
appreciated. Or, if you disagree that this is the most reasonable approach,
please send your suggestions about how you would approach this problem.
Please note, I do understand that the lack of replication is a problem, and
would prefer to have multiple B groups. However, I also note that under
GLM, when data are normally distributed, the assumption of equal variance
allows such an analysis to proceed without even producing a warning.
HOH Fisheries Science Center
HC7 Box 62
Ingram, Texas 78025