What Happens When A Pitch is Thrown: Part 1 - The Strike Zone

This is the first of a series of posts. I'm not sure how many - we'll see where it goes.

Baseball can be described as a series of events. In the last game of the World Series, for example, Boston won 6-1. On a more detailed level, the Red Sox took command by scoring three in the third and three in the fourth, the biggest hit being Shane Victorino's bases loaded double in the third. But at the most granular level, the game was determined by the 284 pitches thrown in the game, and what happened with each of them. We're going to look at how that works.

When I say we're going to look at this, I'm not using the editorial we. I'm going to ask a lot of questions and try to provide answers. Most of the time, I'm finding out the answers for the first time. We're going to find out things together.

So here goes.

How often is a pitch in the strike zone? There are several places to find this. I used Fangraphs, which has customizable leaderboards. I looked up the percentage of pitches in the strike zone. The answer, in 2013, is 44.7%. This isn't the same thing as the percentage of pitches that are strikes. (I'll get to that next.) Sometimes a pitch in the zone is erroneously called a ball. Sometimes a pitch outside the strike zone is erroneously called a strike. Often, a batter will swing at a pitch outside the strike zone. All those can turn pitched strikes into balls and pitched balls into strikes. Here, we're just looking at the percentage of pitches that cross the plate in the strike zone. The answer, as I said, is 44.7%.

Is there a difference between the leagues? Not much of one: 44.7% in the American League, 44.8% in the National League. Maybe it's a little higher in the NL because pitchers don't have to get cute with the strike zone when the other team's pitcher is at the plate; they can just throw it in the zone and there's a good chance the pitcher will make an out. I'm not going to worry about it; the difference is too small.

Is there a difference between starters and relievers? A slight one. Starters are in the zone 44.7% of the time, relievers 44.9%. Again, that's a pretty small difference. I think we can attribute it to two factors. First, relievers come into the game fresh and aren't fatigued. Second, relievers often come into pressure situations where a base on balls is more damaging than, say, a walk in the second inning with one out and nobody on. Anyway, the difference isn't large.

Is there a difference between right-handed and left-handed pitchers? Lefties are in the strike zone on 44.9% of their pitches compared to 44.7% for right-handed pitchers. I'm not going to hypothesize yet on the reason for this small difference. 

Does it matter? This is a key question when looking at any statistic: Context. For instance, last year the Dodgers won 88% of the games that they led after seven innings. Does that mean that if you want to win against the Dodgers, you need to score early, because their bullpen locks down leads after seven innings? No. In fact, that 88% success rate was one of the lowest in the majors. A figure doesn't tell us anything by itself, we need to know what it means.

So what does throwing strikes mean? To answer the question, I borrowed a method from the Bill James Handbook 2014 and divided all pitchers into ten roughly equal-sized groups, based on the percentage of their pitches that were in the strike zone. The top 10% ranges from Wesley Wright, who, in 53 2/3 innings split between the Astros and the Rays, got 48.4% of his 910 pitches in the strike zone, to the Cardinals' Rob Johnson, who threw only four pitches all year with three in the zone. Familiar names in this group include Dodgers closer Kenley Jansen (55.1%), Phillies starter Cliff Lee (53.3%), and Reds workhouse Bronson Arroyo (50.0%). The bottom 10%, all of whom got 41% or less of their pitches in the strike zone, includes Yankees ace Hiroki Kuroda (38.8%) and A's closer Grant Balfour (39.6%). So if there are good pitchers in the best and worst groups, how important is this?

Pretty important, it turns out. Here are ERA, WHIP (walks and hits per inning), and percentage of batters struck out, walked, and allowed home runs for each group:
         ERA  WHIP   %K    %BB   %HR
     1  3.67  1.23  20.2   6.5   2.6
     2  3.66  1.23  20.8   7.1   2.7
     3  3.65  1.25  21.2   7.8   2.4
     4  3.73  1.27  21.1   7.9   2.6
     5  3.88  1.30  18.2   7.2   2.4
     6  3.89  1.30  20.8   8.1   2.5
     7  3.94  1.32  19.2   7.8   2.5
     8  4.00  1.33  20.2   8.9   2.4
     9  4.08  1.37  19.2   8.8   2.6
    10  4.17  1.40  17.9   9.3   2.4

In the table, the first row is the top 10% of pitchers, the second is the next 10%, etc. This says that pitching to the strike zone is important, though not as dramatically as we might've thought. There really isn't much of a difference among the top 30%, other than that pitchers who throw more pitches in the strike zone walk fewer batters. Once you get past them, though, the ERAs and WHIPs start climbing. The dividing line's right around 46% of pitches thrown in the strike zone--below that, and performance starts to deteriorate. The one exception is home runs, which don't appear to be dependent on the percentage of pitches thrown in the strike zone. In fact, the correlation between home run rate and pitches in the strike zone is positive, indicating that pitchers in the zone tend to allow more homers.

What We Know So Far: Pitchers who throw more pitches in the strike zone are generally better pitchers than those who don't.