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From   Timothy.Mak@iop.kcl.ac.uk
To   statalist@hsphsun2.harvard.edu
Subject   st: MIME-Version: 1.0
Date   Mon, 1 Aug 2005 10:28:11 +0100

Hi stata list! 

I've posted this question to allstat before and stata technical support, 
but received no answer. Don't know if you could help: 

Below is the question I posted: 

I have data collected across 4 sites and would like to perform a test to 
see whether the relationship between 2 binary variables, say A and B, 
differs between the sites. 

I think it makes sense to treat the sites as a random sample of sites, and 

thus use a random coefficient model with both random slope and intercept. 
Test would then be via a likelihood ratio test with the nested random 
intercept model (without the random slope). 

1st. My question is: Is this all good? 

Secondly, however, in one of the sites, the dependent variable (B) has 
only one value. So in normal (fixed-effects) logistic regression, this 
would be impossible due to the presence of empty cells. (In other words 
for both levels in A, the outcomes in B are entirely 0 in this particular 
site). 

Modelling using random slopes and intercepts won't give me an error 
message, but I wonder if the output is still valid. 

For your information, my entire dataset consist of 248 observations. THe 
smallest sites still have 28 observations. THe variable A has about half 0 

and half 1 in all sites. THe variable B has about 85% 0 and 15% 1 in all 
sites except the above. 

I used gllamm in STATA for the modelling. What struck me was that for the 
other sites, my unconditional log-odd-ratios (b1+u1) are (.000, .110, and 
.175), but for the site with a pure B variable, STATA gives me a 
log-odd-ratio of .888, considerably higher than the rest. 

****

And also another query: I've written an ado file that lets you draw line 
graphs easily, with dots representing means, and error bars and so on. 
What do I do if I want to share this with you guys? 

Thanks a lot. 

Tim

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