Plate Discipline (O-Swing%, Z-Swing%, etc.)

Plate Discipline statistics tell us how often a hitter swings and makes contact with certain kinds of pitches or how often a pitcher induces swings or contact on certain kinds of pitches. We host a variety of plate discipline statistics on the site and draw from two separate data sources (Baseball Info Solutions and PITCHf/x).

These numbers are very useful for determining the type of hitter or pitcher at which you’re looking and changes in these numbers can often be indicative of underlying changes in a player’s approach.

Calculation:

There are many statistics that fit into the category of “plate discipline,” and the basic definitions are provided below. Please remember that we have data for these from two different sources. The PITCHf/x numbers are raw PITCHf/x data, while the BIS numbers have been modified by human coders so they will not always be in perfect agreement and are subject to some measurement error.

O-Swing% = Swings at pitches outside the zone / pitches outside the zone

Z-Swing%  = Swings at pitches inside the zone / pitches inside the zone

Swing% = Swings / Pitches

O-Contact% = Number of pitches on which contact was made on pitches outside the zone / Swings on pitches outside the zone

Z-Contact%  = Number of pitches on which contact was made on pitches inside the zone / Swings on pitches inside the zone

Contact% = Number of pitches on which contact was made / Swings

Zone% = Pitches in the strike zone / Total pitches

F-Strike% = First pitch strikes / PA

SwStr% = Swings and misses / Total pitches

Here are the 2014 plate discipline leaderboards for hitters according to BIS data, and here are the same leaderboards according to PITCHf/x data. You can also find plate discipline data on player pages and can use the “Averages” button to show the player’s numbers next to league average for that year. Additionally, our BIS data updates nightly during the season, but on a delay, so recognize that the most recent games may not be include when using that particular set of stats.

Why Plate Discipline:

Plate discipline stats are very important for hitters and pitchers because they tell you about a hitter’s approach (or hitters’ approaches against a pitcher). We care a great deal about outcome statistics like BB%, K%, wOBA, etc, but we also want to know some of the underlying factors at play. Is this hitter particularly aggressive? We have a metric that tells you how often he swings at pitches outside the zone, or how often he swings in general.

Are we looking at a swing and miss starting pitcher, or a hitter who has trouble making contact? We have a statistic that tells you how often contact is made when the batter swings.

These statistics won’t tell you the entire story, because a batter who swings a lot on the first pitch because he’s aggressive and one who swings a lot on the first pitch because he happened to see a lot of hittable pitches won’t separate out here, but you can typically get a sense of a hitter or pitcher’s approach by looking at the sum total of their plate discipline numbers. Ultimately, a player can succeed with a wide variety of discipline stats, but knowing these will help you identify the player’s style and skill.

How to Use Plate Discipline:

Plate discipline stats are pretty easy to use because you’re just dealing with raw percentages. As long as you’re clear on the numerator and denominator of each stat (listed above) then you should be pretty well prepared to use these values. You know what an average swing rate is and you can compare it to the swing rate of the player in question.

However, you should remember to use plate discipline numbers in the context of other stats. A 95% contact rate means a very different thing if the player is Juan Pierre compared to Victor Martinez. Lots of contact on pitches outside the zone might be good, but if you’re swinging at lots of bad pitches and grounding out weakly, that’s not a very useful event. Additionally, not all pitches inside the zone are created equal, for instance. Ideally, you should swing at pitches against which you can make solid contact, but that set of pitches is not easily defined by in and out of zone in all cases.

Additionally, you have to be careful with sample size. While these numbers are on a per pitch basis which allows them to return large samples of data over the course of a season, you have to remember that at any given point in time, they are subject to random variation. You can’t look at a player’s last 30 PA, observe a 95% contact rate and assume he’s made a meaningful change in his approach or talent level. By the end of a season, you’re more likely looking at a real trend than if you’re looking at one season of ISO, but that doesn’t mean you can make judgements about players with small samples of these stats either.

Context:

Please note that the following chart is meant as an estimate, and that league-average for all of these stats varies on a year-by-year basis. To see the league-average plate discipline stats for every year from 2002 to the present, check the FanGraphs leaderboards. This table is based on BIS data, but PITCHf/x data is similar. It is important to note that the exact definition of the strike zone varies by source and year.

Stat	Average
O-Swing	30%
Z-Swing	65%
Swing	46%
O-Contact	66%
Z-Contact	87%
Contact	80%
Zone	45%
F-Strike	59%
SwStr	9.5%

Things to Remember:

● These statistics are useful for evaluating hitters and pitchers, with SwStr% being especially important when looking at pitchers.

● Swinging strike percentage (SwStr% — swinging strikes per pitch) should not be confused with whiff rate (swinging strikes per swing).

● The definition of the strike zone varies by source, year, and umpire, which means small differences in these numbers should be expected even if the player behaves the same way.

● While these numbers are useful, they do not take into account the game situation or opposing hitter/pitcher, which will have some impact on how a player behaves.

● While high contact is good for hitters and bad for pitchers in general, there is no ideal plate discipline arrangement. You can be a productive player with a high O-Swing% or a bad hitter with a very high Contact%. It depends on a variety of other factors.

Links for Further Reading:

Plate Discipline Stats – FanGraphs

Introducing Plate Discipline Stats – Hardball Times

Plate Discipline Stats in Action – Hardball Times

The Strike Zone Expansion is Out of Control – Hardball Times