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March 18, 2013

Baseball Therapy

You Gotta Keep 'Em Separated

by Russell A. Carleton

It's the time of year when managers start thinking about games that will actually count. Positional battles are heating up, because decisions need to be made. Opening Day starters are being named. Variations in lineups are being considered, for facing righties, lefties, and Pat Venditte. Your favorite team has spent the spring trying to decide between two players, both of whom are relative unknowns. Due to the 50/50/90 rule (when you have a 50/50 chance of getting something right by chance, you will get it wrong 90 percent of the time), they will pick the wrong utility infielder and the other guy will become a decent starter for some other team.

One thing that managers also have to do around this time is set their starting rotation. Opening Day starters can't pitch all 162 games, Old Hoss Radbourn notwithstanding, and managers need to figure out who will pitch game two, then three, then four, and then the rest of them. In general, the best strategy would seem to be to have the best guy go first, then the next-best guy, then the next-best guy, and so on.

But wait a minute: managers often talk of wanting to structure their rotation such that two similar pitchers never appear back-to-back. You'll often hear managers talk about needing to split their left-handed starters (although the fact that righties pitch back-to-back all the time never seems to bother them). The standard explanation is that if a team sees two lefty starters in a row, it will get used to the spin of a left-handed pitch and be better hitters for it. So, managers take extra care to split their lefties, or their sinkerballers, or their guys with short last names.

Should they?

Warning! Gory Mathematical Details Ahead!
I took all events from 2003-2012. (So, for the initiated, we won't be having any statistical power problems, will we?) I looked at all starting pitchers who faced at least 250 batters in their capacity as starters during that year. I classified each by three criteria: handedness was the easy one. The other two I borrowed from baseball-reference.com: power vs. finesse pitchers and ground ball vs. fly ball pitchers. I grouped all pitchers within each year by whether they were in the top third, middle third, or lower third of the percentage of their batters faced who struck out or walked, for the power/finesse dimension. Then I did the same three-group division for their ratio of groundballs to fly balls. Chopping any continuous variable into discrete groups is always risky, but in the interests of putting together pitcher types, this will have to do for now.

For all games, I figured out who the starting pitcher that the other team had seen the night before was, in terms of our three criteria, and whether the pitcher was a match on any of the three criteria (and also if he was a match on all three). The "match" variable was coded dichotomously (aka, 0 and 1).