And finally, the numbers go all the way back to 1871 for hitters and 1876 for pitchers. Some of the estimates used to fill these stats in, like the baserunning regression formula or the JAARF fielding numbers, are not to be trusted as anything more than a reasonable guess. Catcher defensive ratings are based on passed balls and errors only. It is not worth it to even try to estimate performance against the running game by catcher assists. Mike Piazza had about as many assists per game as Johnny Bench. Enough said.
Here’s the WAR database.