Hello. I hope you can help.

I am trying to build a conditional logistic regression model to analyse a 1:4 matched nested case control study. All cases were matched on 5 year age band, sex and were all active in the same GP practice during the same period.

I understand that due to the risk of bias I need to adjust for the matched factors in the model.

The problem I am encountering is that because sex is the same between case and the 4 controls (because it is a matched factor), so when I try to include it in the model, it is "omitted because of no within-group variance." I understand that this is happening because the case and each of their four controls all have the same value and are not therefore a discordant group- so are not included in the analysis. How then would I adjust for sex in my model, without the variable being dropped?

I am also unsure what to do about the age bands. I have the exact date of birth for all cases and controls but they were matched within 5 year bands. Should the age variable which I adjust for in the model use the actual date of births for all cases and controls even though I matched within 5 years? or should I code all controls as the same date of birth as the case which they are matched to- in which case I would surely run into the same problem as earlier, as all my groups would be concordant and therefore dropped from the analysis.

Many thanks!