Hey mariamb.

Can you use whatever software/data-environment you have and select the subset of data you are interested in and analyze this particular subset? (In other words treat it as a conditional distribution).

If you are doing a conditional analyses, you will need to think about how the degrees of freedom will change as a result (i.e. do you have enough data to be able to fit a particular model?).

If you are using something like R or SAS, getting the subset data should be straight-forward (for SAS use the DATA command and R you can use any of a tonne of procedures like data.frame()).