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And we need to understand that there are some phenomena in baseball that are so complex that we'll never be able to isolate the variables on them.
There are simply too many moving parts for us to confidently measure and find out whether it was Roger Clemens' size that allowed him to throw 250* innings a year with no problems.  You can't control for things like his arm angle, the inverted W, the acceleration of his IP load, the mounds he threw off of with the footing, and literally >100 other things.
I suspect that catcher impact on ERA is just too buried in other variables to ever isolate it.
............
This is part of the reason that it's so unfortunate for the BP author to have the belief, "If I can't measure it, in practical terms that's the same thing as it not existing.  If I can't measure it, it doesn't matter."
Very harmful worldview.