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One of the great missing adjustments to even our most advanced stats is the adjustment for the quality of the opposition. A given stat line put up in the AL East means something rather different from the NL Central. Compounding the problem is that the adjustment needs to be iterative; once you have calculated the opposition quality for everyone and adjusted all the stats, you have to paste the adjusted stats over the originals, recalculate the opposition quality, and so on, again and again until the values stabilize. I do that with my adjusted standings at SoSH, but no one, AFAIK, has ever done that with individual hitting and pitching lines.
How exactly would you do this? You could just take opponent overall quality, which would in fact be your best adjustment for value. But an interesting and in some ways better alternative would be to include handedness. For instance, each LHB would get an adjustment based on the numbers versus LHB of the LHP pitchers he faced, and a separate adjustment for the RHP.
(As an aside, if you’re studying whether some hitters have persistent quality-of-opposition splits, you have to do it this way. AFAIK, no one ever has — the few studies showing no such persistence have used opponent ERA, which adds a huge amount of noise. Jon Lieber in his prime was a great pitcher vs. RHB and a lousy one vs. LHB; why count him as average vs. everyone?)
This notion of adjusting for opposition quality by handedness immediately suggests a value adjustment I’ve never heard mentioned. As a rule, elite LHB face a better quality of LHP than do average LHB. Not only are they more likely to not get benched against a C. C. Sabathia, they are hugely more likely to face a nasty LHR in the late innings. The differences among LHB are mitigated (slightly? more than that? I don’t think anyone knows) by this. The quality-of-opposition adjustment I just outlined would put the proper distance between the Alex Coras and Adrian Gonzalezes of the world. And this would be hugely desirable and interesting — when you’re assessing talent, that is.
Now, the funny thing is this: when assessing value, this can probably be safely overlooked. It’s built into the way the game is played now that this will happen. That we are underestimating how much better Gonzalez is than Cora is pretty much negated by the better pitching Gonzalez faces as a result.
However, there is probably a small but very interesting class of exceptions to this rule. You would expect there to be some hitters who get too much or not enough respect from opposing managers, and thus face more or fewer LHR than they ought to based on their own platoon splits. You would adjust for this by finding the correlative relationship between LHB platoon splits and the percentage of time they are at the platoon disadvantage, and then calculate the expected number and quality of LHR they faced versus the actual. The players with the biggest differences, in both directions, would make for very interesting lists. It’s possible that some of the “noise” in platoon splits is actual signal; as LHB establish reputation, managers begin to match them up with their lefty-killers. But reputations lag behind reality, both at the start and end of careers (David Ortiz may now be seeing tougher LHP than other LHB of his quality).
(As an aside, I know that Trot Nixon’s career path of splits vs. LHB was made completely nonsensical by the genius of Jimy Williams, who benched Nixon against even the easiest LH starters but never pinch hit for him against even the toughest LHR. So he was probably leading the league in toughest quality of LHP faced despite being nowhere near the top of the list for overall LHB quality. That’s the sort of guy it would be neat to identify and adjust.)