There is someone I play with quite a bit. Quite often I give him position. Hence, I think he tends to play a lower VP$IP/PFR when I'm at the table because I make it uncomfortable for him to enter pots light.

Also, post-flop reads are far easier to make against loose players than tight players for obvious reasons.

Anyway, you don't need to be scientific about it. Just look at the sample size, adjust a little for looseness and take the data for what it's worth along with the dozen other little pieces of information that feed into a good poker decision.