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st: comparing two ICCs


From   VISINTAINER PAUL <VISINT@NYMC.EDU>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: comparing two ICCs
Date   Tue, 18 Jun 2002 15:22:41 -0400

I wish to compare two intra-class correlations: males vs. females.  Each
group has a sample of 13 persons, with 3 measurements on each person.  The
-loneway- program provides the following estimates:

male:    ICC=.90,  s.e.= .046   95%CI: .809, .990
female:  ICC=.69,  s.e.= .120   95%CI: .459, .929

The manual states that the confidence intervals are computed as:

       (rho-1.96*se , rho+1.96*se).  

This indicates to me that the confidence intervals are computed under the
assumption that rho is normally distributed.  If this is valid, it would
seem appropriate to extend the assumption to provide for a simple method to
test the difference between the two ICCs:

e.g., z = (icc1 - icc2)/sqrt(se^2 + se^2).   (Or more conservatively, to use
the t-distribution, rather than z).

Does this make sense?


Paul Visintainer
New York Medical College






Paul F. Visintainer, PhD
School of Public Health
New York Medical College
Valhalla, NY  10595
(914) 594-4804  (phone)
(914) 594-4292  (fax)

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