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February 11, 2014

Baseball Therapy

When Sabermetrics Gets Personal

by Russell A. Carleton


Let’s try something.

In the recently released 2014 edition of the Baseball Prospectus annual, I wrote an essay calling for a new kind of analysis. The field of sabermetrics, as a whole, has spent a lot of time trying to figure out larger truths about baseball players in general. There’s nothing wrong with that. We’ve found some good ones. But just about every study begins with finding every single player who fits some criteria and has a big enough sample size to produce reliable data. That allows us to say things about baseball players as a group. In the annual essay, I tried to make the case that it’s a little strange that there is no parallel track of research for the study of individual players. It’s time we got a little more personal.

One topic that I’ve done some research on is the question of how big a sample size we need for a stat before we can stop saying “small sample size.” The answer varies from stat to stat, both for pitchers and hitters. But the idea is that there comes a point—for example, 60 plate appearances for strikeout rate—where you can believe that a hitter’s strikeout rate over those 60 PA really is an adequate reflection of what his talent was at the time. That’s the base rate for hitters in general, but does that hold for this particular hitter?

People often misinterpret the thresholds for various metrics to stabilize, believing that 60 PA means, “Now that we’re past 60 PA, from this point forward, we can believe that Smith will maintain this new strikeout rate.” It’s not a bad assumption, but it doesn’t always work like that. Maybe he will, maybe he won’t. Smith might make some sort of adjustment mid-season, maybe a radical one. Baseball players are human, and many of them make changes in their lives. Some maintain a fairly consistent approach all year. (Which one is better is an empirical question.)

In some sense, when a player makes a change, he’s become different player. Maybe it’s just a slightly different player, but so far we have lacked the vocabulary, statistical or otherwise, to describe those differences within a season. Baseball statistics come in many flavors, but they are almost always denoted in whole-season format. A .300 hitter may have been a true talent .300 hitter all year, or perhaps was a .290 hitter for half the year and a .310 for the rest of it. How can we tell who’s who?

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Premium Article Barry Bonds, Race, and... (02/11)
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