[jacquesm]

If they use it for statistical analysis then the sample size decrease a bit, but likely not in a significant way. If they lost all their data that would be a bit different.

For instance, I routinely delete web server logs older than 30 days, on the assumption that if I didn't need it in the last 30 days I'll probably never need it. Every now and then this bites me and I need more than 30 days data to test some assumption, then I will just have to wait for a bit. (for events that occur infrequently enough).