In a related question, I asked about individual versus aggregated data and received a very informative response:

http://www.listserv.uga.edu/cgi-bin/wa?A2=ind0808b&L=sas-l&F=&S=&P=8873

As I am dealing with person-time data, I have a related question of degrees of freedom. Assuming independence, or stipulate it for the sake of argument, what would be better for a study:

1) Observations on 500 subjects for 1 year each 2) Observations on 300 subjects for 2 years each

Let's further assume that these subject have been matched, that is have the same covariate values, so that information (combinations of covariates that allow for estimability) is not an issue.

I would, perhaps naively, prefer #2. Given some of the conclusions in the post above, namely that the SE and differences in LL for the LRT are the same, correctly specifying the DoF may not be critically important.

At least for an exercise, it interests me. If we defined the rate to be outcomes per person-year, then might we weight the subjects in #2 to reflect their person-years, i.e. 2?

A sensible follow-up would be what happens with non-integer results: 4.5 person-years, for example?

Any insights, references, critiques, corrections, or flames are greatly appreciated.

Kind regards,

Kevin