What Happens When a Pitch is Thrown: Part 3 - Swing, Batter

So far in this series we've seen that while it's good for pitchers to throw strikes, it's better to get strikes. The difference between pitching a ball into the batter's strike zone and getting the batter to swing at the pitch (or get it called a strike) is largely a function of inducing swings on pitches that aren't in the strike zone. We'll look at that now. 

How often do batters swing at pitches? Batters swung at 46.4% of pitches last year. That compares to 44.7% of pitches that were in the strike zone. Batters swung at 65.5% of pitches in the strike zone and 31.0% of pitches outside the strike zone. Don't those figures strike you as low and high, respectively? But when you think about about, it makes sense. Super-selective hitters let a lot of strikes go by as they wait for something they can hit. Mike Trout and Joe Mauer each swung at fewer than 56% of the strikes they saw, while even the most swing-happy guys have some strikes called on them. As for the outside-the-zone pitches, some pitches just an inch or two out of the strike zone may be to a hitter's liking, while some breaking balls with late movement fool batters as they dart out of the strike zone.

Is there a difference between the leagues? Yes, a slight one. NL pitchers get more swings. Batters facing AL pitchers swung at 46.0% of pitches: 65.0% in the strike zone and 30.7% outside the zone. Batters facing NL pitchers swung at 46.8%: 66.0% in the zone and 31.3% out of it. I thought, "Must be pitchers batting," but pitchers watch a lot of strikes go by. Last year they swung at 46.1% of pitches: 57.4% in the zone and 33.8% out of it. So NL pitchers get more swings outside the strike zone in part because of pitchers flailing away when they bat. But the reason NL pitchers get more swings in the strike zone is due to something else.

Is there a difference between starters and relievers? Relievers get more swings. Starting pitchers got batters to swing at 46.4% of pitches: 65.4% in the strike zone and 30.9% outside the zone. Relievers got swings at 47.7% of pitches: 67.2% in the strike zone and 31.9% outside the zone. Relievers, as I've mentioned earlier, come in and throw gas for an inning or so, so they get more strikeouts, almost one more per nine innings than starters. 

Is there a difference between right-handed and left-handed pitchers? Lefties got batters to swing at 46.3% of their pitches: 65.0% in the strike zone and 31.0% out of the zone. For righties, pitchers got swings 46.5% of the time: 65.7% in the strike zone and 31.0% out. The conclusion here is that left-handed pitchers induced fewer swings on pitches in the strike zone than righties, which means they must've gotten more called strikes. I have no idea why that should be. The difference isn't large, so I'm not going to worry about it.

Does it matter? Let's look at swings in the strike zone and out of the zone. First, in the strike zone. As with the previous entries, I've divided pitchers into then groups: the 10% who got the most swings in the strike zone (Group 1), the next 10% (Group 2), the next 10% (Group 3), etc. I looked at how the groups compared in ERA, WHIP (walks and hits per inning), and percentage of batters struck out, walked, or allowed a home run:
   Group ERA  WHIP   %K    %BB   %HR
     1  4.03  1.33  19.8   8.3   2.7
     2  3.45  1.22  20.7   7.4   2.4
     3  4.14  1.31  19.8   7.7   2.9
     4  4.18  1.36  18.1   7.7   2.6
     5  3.59  1.25  19.9   7.3   2.3
     6  3.87  1.30  18.6   7.7   2.6
     7  3.66  1.29  19.6   7.9   2.2
     8  4.11  1.32  20.2   7.9   2.6
     9  3.76  1.29  20.2   7.9   2.4
    10  3.92  1.34  21.9   9.5   2.5
Do you see a pattern there? I don't. I think we can safely say that getting batters to swing at pitches in the strike zone isn't correlated to pitching success.

How about pitches outside the strike zone? That's a different story completely:

   Group ERA  WHIP   %K    %BB   %HR
     1  3.20  1.13  22.0   5.8   2.3
     2  3.44  1.22  21.5   7.2   2.4
     3  3.67  1.25  20.6   6.9   2.5
     4  3.83  1.27  20.6   7.4   2.4
     5  3.73  1.27  19.8   7.3   2.4
     6  4.00  1.29  20.3   7.8   2.9
     7  3.85  1.31  19.3   8.4   2.4
     8  4.15  1.36  18.4   8.3   2.8
     9  4.44  1.47  17.2   9.6   2.5
    10  4.49  1.47  18.6  10.7   2.6
This is almost as dramatic as the impact of getting strikes, detailed in the last post: The pitchers who got the top 10% of strikes had an ERA of 3.16 and a WHIP of 1.10. Group 10 had an ERA of 4.64 and a WHIP of 1.55. It's the same pattern for pitchers who get batters to swing at pitches outside the strike zone. The best pitchers get the most strikes, and they get strikes in part because they throw pitches in the strike zone but also, at least as importantly, because the get batters to swing at pitches outside the zone.

I looked at pitchers who do one well but not the other. First, pitchers who were in the top 30% of throwing pitches in the strike zone but the bottom 30% of getting swings outside the zone. The numbers are:
              ERA  WHIP   %K    %BB   %HR
             4.14  1.36  18.8   8.3   2.9
     Average 3.87  1.30  19.9   7.9   2.5

(Average is the major league average last season.)

Now, pitchers who were in the top 30% of getting batters to swing at pitches outside the strike zone but the bottom 30% of throwing pitches in the zone. The numbers for them are:

              ERA  WHIP   %K    %BB   %HR

             3.55  1.25  20.1   7.6   2.2

     Average 3.87  1.30  19.9   7.9   2.5

I wasn't expecting that. Pitchers who are good at throwing pitches in the strike zone but bad at getting batters to swing at pitches outside the zone are below-average pitchers. Pitchers who are bad at throwing pitches in the strike zone but good at getting batters to swing at pitches outside the zone are above-average pitchers. Pitchers in the former group include Rockies starters Jhoulys Chacin and Juan Nicasio, Yankees closer-in-waiting David Robertson, free agent innings eater Bronson Arroyo, and Dodgers washout Brandon League. Pitchers in the latter group include Angels starter Jered Weaver, Yankee starter Hiroki Kuroda, Pirate ace Francisco Liriano, and several top closers, indluding Jason Grilli, Mariano Rivera, Joe Nathan, and Sergio Romo. 

What We Know So Far: The best pitchers get the most strikes, in part because they throw pitches in the strike zone but also, more importantly, because the get batters to swing at pitches outside the zone.

Next, we'll see whether these rules apply to hitters as well.