From a current article on Bill James Online entitled ” Xavier University Business School Speech”:
“One of the things that we actually do in my field is to try to straighten out the misconceptions that people have, based on misunderstanding the statistics. One of the largest of these, actually, has to do with the platoon differential. The platoon differential, for those of you who are not baseball fans, has to do with the fact that left-handed hitters hit better against right-handed pitchers, and right-handed hitters hit better against left-handed pitchers.
But what almost nobody understands, inside of baseball or outside of it, is that this is not an individual variable. It’s a condition of the game. It applies to everybody.
But we MEASURE what hitters hit against left-handed and right-handed hitters based on individual hitters, year by year. Because we are dealing with a relatively small advantage and relatively small data groups, the data for individuals is all over the map. A right-handed hitter may hit .260 against right-handed pitchers, but .420 against left-handers. That’s called the platoon differential. This SEEMS to be tremendously important, and, because we measure it on an individual basis, people ASSUME that it is an individual trait.
But it actually isn’t; it’s a condition of the game that applies to everybody. There isn’t actually any such thing as a hitter who hits .420 against left-handed pitchers, or a hitter who has a 160-point platoon differential. That’s just a statistical fluke, just like the guy who hits .420 on Tuesdays. The platoon differential is basically the same for everybody.
But baseball people don’t understand this; they didn’t understand it 30 years ago, and they don’t understand it now, so they talk literally every day and many times every day about platoon differentials, always assuming that it is an individual variable. Because they grow up with that understanding, and because that understanding is re-enforced every day by selective observations, it takes a long, long time to get people to understand that there is no such ability. But we will eventually succeed. A hundred years from now, people will generally understand that there isn’t any such ability, and never was.
I should be careful about speaking in absolutes. There actually is at least one major league player who has an abnormal platoon differential. Adam Jones of the Baltimore Orioles is a right-handed hitter who actually does hit better against right-handed pitchers than left-handers. But he is probably the only one. “