What a Smaller Strike Zone Can Do for Pace of Play

Last week, I discussed the consequences of an expanded strike zone on the game, finding that it leads to more strikeouts and fewer balls in play. While some have suggested that a larger zone — by inviting more swings from batters — might actually result in an uptick in batted balls, the observed results don’t support that hypothesis. Whatever gains a larger zone creates in terms of swing rate, they’re negated by an increase both in whiff rate and called strikes, leading to more strikeouts overall.

What that post addressed was what would happen if the strike zone got bigger. This post attempts to answer a similar question — namely, what would happen if the strike zone got smaller?

In order to test the effects of a shrinking strike zone, it’s necessary first to identify an actual instance in which the strike zone has gotten smaller. Fortunately, such an instance exists, thanks again to the research of Jon Roegele, who produced this visual in his piece on the strike zone last year.

That’s the 2007 strike zone on the left and 2017 zone on the right. As you can see, the outside edge to lefties used to be called a lot more frequently than it is now. The bottom of the zone has gotten larger for both lefties and righties (a point addressed in my last post), and the result has been a smaller strike zone for lefties than their right-handed counterparts.

To measure the effect of the changing zone, we will again use the same zones from Baseball Savant, which looks like this:

In this case, we will focus on Zone 14, where the zone has seemingly shrunk over the past decade. Again, these results are based on a number of searches at Baseball Savant. The graph below shows the percentage of called strikes in the zone as well as the percentage of called strikes among pitches taken.

We can pretty easily see that fewer strikes are being called when hitters take a pitch in Zone 14. The effect is present in overall called strikes, as well, but not as strongly. We aren’t actually capturing all of the old lefty strike zone, as it actually went well past the edge in Zone 14. Back in 2011, 15% of pitches in Zone 24 were called strikes compared to just 3% this year. That means the result of the smaller strike zone is more pronounced than shown above, but by looking at performance in just Zone 14, we should be able to get a decent idea about how a smaller strike zone affects balls in play.

The graph below shows how often pitchers have pitched to the outside edge against lefties.

In 2011, 12.3% of pitches to lefties were located on the outside edge; this year, that number has fallen to 10.7%. That 1.6-percentage-point change might not, at first blush, seem significant, but it does make a difference. Left-handed batters account for roughly 40% of all plate appearances and have recorded a swing rate above 50% in that area. So even with a relatively small change in number of called strikes, the result is one fewer called strike per game and one batted ball every other game.

Losing a called strike and one half of a batted ball might not seem like the desired outcomes. Fewer strikes leads to more walks, and the idea here is to increase batted-balls frequency, not reduce it. This is why it’s also important to consider where pitchers are throwing instead of that outside edge. If you look at the strike-zone map from above, you see Zones 1-9 are the middle of the strike zone. If the desired outcome is an increase in balls in play, those are the areas where pitch frequency needs to increase. Pitches taken in those areas are strikes 98 out of 100 times, hitters swing at the nearly three-fourths of the time, and when they swing, a ball is put in play 45% of the time.

In 2011, pitchers threw the the ball 25.9% of the time to zones 1-9 when facing lefties. This season, that number is up to 27.9%. We could chalk that up just to more aggression from pitchers, in general, but while pitches to lefties in the zone have gone up by two percentage points, pitches to righties have gone down there by roughly a full percentage point. Lefty batters are now swinging at a greater percentage of pitches in the zone than they did in 2011, which means the number of called strikes hasn’t changed, but the number of balls in play has gone up by roughly one per game, counteracting the half-a-game loss of a batted ball due to pitchers targeting the outside edge less frequently.

It’s reasonable to wonder how a 1.6-point decrease on the outer edge of the zone could lead to a two-point increase in the middle of it. That seems unrealistic. Here, though, it’s necessary also to look at the pitch frequency in the area just past the outside edge. I mentioned earlier that pitches in Zone 24 went from being called a strike 15% of the time to just 3% of the time. That’s also an area that induces a batter to swing just around 20% of the time, and only one-third of swings result in a batted ball. That area has seen a major reduction in the frequency of pitches thrown to lefties, going from 11.0% in 2011 to just 7.7% this year. Pitchers can’t get strikes out there like they used to, so they target that area way less than in the past.

The net result has been more balls in play for lefties compared to righties, whose strike zone is larger. Since 2011, strikeouts against right-handers have gone up 4.2 percentage points while strikeouts against left-handers have gone up just 3.3 percentage points. Meanwhile, walks have gone up 0.9 percentage points for left-handers but just 0.3 percentage points for right-handers. While we do see a few more walks, the end result is fewer strikeouts. Because the decrease in strikeouts is greater than the increase in walks, we see more balls in play.

Shrinking the strike zone forces pitchers to adjust, and they do adjust — by throwing more pitches in the middle of the strike zone. While there is a small increase in the number of walks, strikeouts decrease in larger numbers, leading to more balls in play. For those who have concerns about the number of balls in play, there is some evidence to think that making the strike zone smaller will lead to a greater number of plays out in the field.

Craig Edwards can be found on twitter @craigjedwards.

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Mo Vaughn
4 years ago

It seems like the league is A) increasingly aware of the higher number of strikeouts and lower number of balls in play, and B) doesn’t believe those things to be good for the game. I think that shrinking the zone is inevitable, and with robot umps, they could experiment with small tweaks to the zone that were immediately consistent league-wide.

4 years ago
Reply to  Mo Vaughn

The purpose of shrinking the zone is to force pitchers to adjust and throw more hittable balls. Experimenting and making small tweaks wouldn’t be effective unless MLB (a) informed all pitchers before the change, or (b) game the adjustment enough time to affect pitcher’s habits on pitch location, which I’m guessing would be close to a month, or more.

4 years ago
Reply to  Joe

You may very well find that pitchers can’t throw balls inside of a shrinking zone. It would possibly be a walk-fest. I think even a moderate change to the zone would shove a whole bunch of guys out of MLB.