st: -xtmixed- and differences in test-retest reliability

[Gillian.Frost@hsl.gov.uk]

Hello all,

I was wondering if anyone would be able to help me with a problem I have. 
I know that there must be some way to do this, but I cannot for the life 
of me figure out how to do it.

A brief explanation of data:
Altogether there are about 40 subjects.  Each subject underwent a test to 
assess their hearing on three different days, three times each day.  The 
goal is to assess the reliability of the test (its reproducibility), as 
well as any between day or within day variation in test results.

I know that I can use xtmixed to estimate a variance-components model, the 
results of which can be used to estimate the test-retest reliability, the 
between days (within subject) intraclass correlation coefficient (ICC), 
and the within day ICC.  I would use the following model:
xtmixed result || subject: || day:

Now here is where I am getting confused.  What if I also wanted to know if 
the test-retest reliability differed depending on some other factor?  For 
example, what if (for some reason) the test was more reliable for males 
than females?  Or more reliable for older age groups than younger ones? 
How would I test for this?  I suppose I could use separate models for 
males and females, but how could I then test whether the ICCs were 
statistically significantly different?

Any help with this matter would be greatly appreciated.  Please just ask 
if anything needs clarifying.

Many thanks,

Gillian



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