Sullivan: Opposition quality seems hardly a factor

From Jeff Sullivan at FanGraphs on February 4, 2015:

Remember when you thought baseball statistics were easy? Now, analytically, a statistic is hardly worth anything if it’s left unadjusted. You know all the adjustments that go into numbers. Adjustments for ballpark environment. Adjustments for era. Sometimes adjustments for league. As far as WAR is concerned, there are adjustments for position. There’s one adjustment we still don’t make, though: that’s adjustment for quality of opposition. In theory, if there were a pitcher who only ever faced the best teams, and a pitcher who only ever faced the worst teams, that wouldn’t be accounted for. That’s something you’d have to figure out yourself.

Related to this, James Shields has obvious selling points: first and foremost, he’s been good. He’s been durable, and he’s experienced, and he’s pitched in the playoffs, and everything. Then there’s one other thing I don’t think has gotten much attention: Shields has, relative to the average, faced a fairly tough slate of opponents. In the past, I’ve manually calculated average opponent wRC+. Baseball Prospectus has its own version, oppRPA+, and the big advantage of oppRPA+ is it’s already been calculated for me. The average, as usual, is 100. Last year, Shields’ opponents came in at 105. The year before that, 105. The year before that, 105.

Seems like this should be a good thing for Shields’ market; seems like, if you adjust for this, Shields’ numbers would get a boost. But how much does this matter, really? Coming up soon, a first attempt at an answer.

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Originally published: February 5, 2015. Last Updated: February 5, 2015.