One of our recent projects at Football Outsiders is a similarity scores system for defensive players, similar to the one we use for offense. The similarity scores judge players not just on standard stats (sacks, interceptions) but FO stats (average yards on tackles, stop rates) plus age, career experience and size. The formula is still in flux, but the results produced are always interesting. Looking at historically similar players can often be a good guide to what we can expect in the future, although each of these players, of course, had a different history of injuries and played in a different scheme with different teammates.
4hEthan Sherwood Strauss