Concerning what we can measure, there just isn’t that much difference between catchers. I’ll do my best to recall it off the top of my head.

Essentially, I came up with seven areas where catcher’s defense comes in to play:
1. Controlling the running game
2. Calling/framing pitches
3. Catching pitches (preventing PB/WP)
4. Catching pop-ups
5. Fielding bunts
6. Fielding throws/blocking plate/tagging runner
7. Dealing with pitching staff

1. The first one turns out to not be that meaningful because primary responsibility likely falls on the pitcher and even ignoring that aspect, there’s a high degree of diminishing returns from the catcher’s POV in preventing SB attempts since stolen bases are not a great tactical move for the offense. The +/- spread per season on this ended up around 5 runs.

2. The second one, a.k.a. cERA, has never, to my knowledge, been demonstrated to be meaningful. I do actually subscribe to the theory that catchers can impact pitch sequencing , and I certainly believe pitch sequencing matters, but I am not confident that there’s a demonstrably better or worse system that can attribute the skill to the catcher. The decision ultimately lies with the pitcher.

3. Quantified already. The +/- spread per season on this ended up around 5 runs.

4. The vast/entire majority of pop-ups that would be fielded by the catcher have so much hang time that there’s such a high success rate that there’s not enough of a sample to differentiate value here. JI’s Clement GIF notwithstanding.

5. Pretty small sample of plays and once again, most just involve throws to first base and made with a high rate of success.

6. Even smaller sample of plays and entirely subjective and depends a lot on the nature of the incoming throw. Possible that it might be quantified, but doubt there’s much spread at all once proper regression is factored in.

7. //shrug