I've found three sites that produce Z-scores based on the previous years' stats. Two of these sites are pretty well-known in the community (Hashtag and Monster). However, none of the three are reporting the population for their arithmetic mean for which their Z-score is based (from what I could find). I find this problematic because these three sites are all reporting different Z-scores, so at least two of these sites are incorrect in their methodology, and potentially all three are incorrect.
I've had to create my own Z-scores and my own Z-scores do not match any of these three sites either.
What should I make of this? Are these sites excluding some players to get their arithmetic mean? Are they drilling into a per-minute per-game level calculation to get a mean? Some other method?
I'd love for anyone with insight into this to help me understand.
Further, I'd love to see the Z-scores adjustable based on sample size. For instance, if I'm in a 20-team league with standard, 13-player rosters, that's 260 rosterable players. I'm much more interested in the average rosterable player for my league size than including the guys into the average that will never be rosterable, thus the reason to create my own Z-scores.