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June 1, 2005

Lies, Damned Lies

Strikeouts and Hitter Projections

by Nate Silver

Here's a secret: strikeouts are a good thing for a young power hitter.

That quote is mine, from a chat session about four weeks ago. It's pretty tempting to come up with punchy, ill-considered one-liners like that when in the midst of a chat, and indeed the comment triggered plenty of e-mails, as well as a persuasive counter-argument by Rich Lederer. So let me try and explain myself a little bit more thoroughly.

My comment was based on something I discovered when revising PECOTA a couple of years ago. Specifically, I found that when everything else is held equal, higher strikeout rates have a somewhat positive predictive effect on power output. For example, the regression equations I use for PECOTA suggest that--all else being equal--45 extra strikeouts in the previous year are "worth" about one additional home run in the upcoming year. The effect is not enormous, but it's there, and it's one reason why folks like Adam Dunn and Hee Seop Choi and Wily Mo Pena tend to get such favorable PECOTA projections.

Strikeout rates also have an inverse predictive effect on base hits, and consequently on batting average (to be a bit more specific, a player who strikes out more can be expected to have somewhat fewer singles; there is no discernable impact on his predicted rate of doubles or triples). This is to be expected. While hitters' batting averages on balls in play do not exhibit the same strong regression to the mean that pitchers' BABIPs do, hitter BABIP is at least somewhat a matter of luck, and you can't get lucky if you don't put the ball in play.

Finally, all else being equal, strikeouts have a positive predictive effect on walk rate, as both strikeouts and walks tend to result from going deep into the count.

Further complicating things is that these general principles can also have some quirky effects depending on the particular characteristics of the player. For example, PECOTA predicted Albert Pujols, a hitter who does not strike out very much, to have a batting line this year of .334/.419/.633. If we go back in the system and double Pujols' previous strikeout rates, PECOTA instead comes up with a projection of .325/.411/.633. Pujols' expected BA has declined by about 10 points; he makes up for some of this with a 10-point increase in his expected isolated power and a slight increase in his "isolated walk rate," but the overall effect on his value is negative. PECOTA is happy, in other words, that Pujols does not strike out very much.