Stats question: Comparison of the same individuals during two exposure times

Hi,

I'm hoping that someone will be able to help.  I would like to compare how
covariates associate with the risk of a binary outcome during two periods. 
Period 1 will be non-exposure to a treatment and period 2 will be exposure
to a treatment.  The same individuals will be examined in each group but I
want to be able to compare the association of certain covariates between the
two groups to see if there is a treatment interaction.  I've looked at
case-crossover designs and time series analysis and don't think that they
are suitable.  The cohort has longitudinal data so individuals will go onto
treatment at different times and the effect of the treatment needs to be
administered for a while before it has an effect.  The reason why I cannot
just go ahead with an exposed vs unexposed design is that most individuals
in the cohort end up on the treatment eventually and the unexposed group is
very small and lacks power for a meaningful comparison.  

Is there anyway to compare the same individuals during different exposure
times and to look at the effect of different covariates under the exposed
and unexposed conditions?

Thanks for you help,
Natalie

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Natalie Van Zuydam

PhD Student
University of Dundee
nvanzuydam at dundee.ac.uk
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