The Adam Dunn debate: Defining plate discipline
May 24, 2007 15 Comments
A small brag, if I the reader will humor me. A study of mine on strikeouts and walks has been published in By The Numbers, which is the official newsletter of SABR‘s Statistics and Analysis Committee. The study (which you can find in this PDF file published under my… umm… real name… shhhhh) is entitled “Is Walk the Opposite of Strikeout” and I argue that the answer is no. Walk and strikeout are actually more alike than similar and that the opposite of these two is “ball in play.”
My starting point is the Adam Dunn debate as to whether he is a “disciplined” hitter. Dunn is the paragon of the “three true outcomes” hitter. He’s hit at least 40 HR in the last three years, while averaging something like 170 strikeouts and 110 walks. In 2006, he had one of those three outcomes in more than half of his plate appearances. Because of the large number of walks that he draws, Dunn has been referred to as a very disciplined hitter by some (probably all those who just finished Moneyball), while others have called him undisciplined, for the obvious reason that he’s been flirting with 200 strikeouts over the past few years. Who’s right? Well, a lot comes down to how you define plate discipline.
The most common measure of plate discipline that I’d found is some sort of ratio between strikeouts and walks, usually K/BB. The problem, which I point out in my article is that there is more than one way to avoid striking out. A walk certainly is one, but is it necessary that players with low walk totals are undisciplined. Is it not equally disciplined to take a big fat hanging curve ball and place it in the left field stands?
To that end, I developed two new metrics based on signal detection theory while taking advantage of the pitch-by-pitch data in the Retrosheet data files. The article in By The Numbers goes into greater detail on how the metrics are calculated, but the basic idea behind signal detection theory is this: A batter must see whether or not the pitch is hittable (i.e., in the strike zone… more on the obvious objection to this in a minute) and then decide whether or not he should swing. He might swing at it and miss, or he might not swing and have it be a called strike. Either way he’s made a mistake and will have a strike called against him. However, from a signal detection theory standpoint, the type of mistakes he makes are telling. In fact, through looking at the two types of mistakes, plus the times that he actually hits the ball or takes a called ball, signal detection theory can generate two measures. One looks at how likely a batter is to swing, the other at how good a batter is at reading the strike zone and making good decisions. He may be good at reading the strike zone, just too anxious (or too passive) with his swings. Or, he may simply be guessing in the strike zone.
It turns out that the two measures actually correlate differently to walk and strike out rate. How good a hitter is at reading the strike zone was very correlated with his strikeout rate, but not walk rate. How likely he was to swing was more correlated with his walk rate, but not strikeout rate. Players who swung less walked more. Looks like walks and strikeouts are manifestations of two different skills.
There are a few problems. One is that players do swing at pitches in the strike zone and miss them. The other is that they will sometimes golf a hit off of their shoetops. Retrosheet data (which is free!) doesn’t give pitch locations. Eventually, I’ll learn how to mine the data from MLB’s Enhanced GameDay to my advantage on this one. But, for now, I think it’s an enhancement over the simple K/BB metric we have now.
In any case, take a look at the article, and discuss. I’d like to refine this a bit and it’s always better to have a little bit of collaboration.
And for what it’s worth, Adam Dunn finished #401 last year among all hitters with at least 100 PA. Out of 431. Perhaps you can tell which side of the debate I’m on.