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Get the all time leaders in WAR or WS or PCA (my own) and rank them by position. What multiplier do you need to get the best ten catchers to have a similar value distribution as the best ten first basemen?
Answer? Roughly 0.4 from PCA - 0.35 if you just want the top four catchers of all time to make the top 40 position players of all time.
I don't think catcher attracts inherently less talented people to it. Which implies that the position adjjustment (career) is roughly to reduce potential value by 35-40%.