Below is a plate discipline graph for hitters in 2010. The x-axis represents swings out of the zone, and the y-axis represents swings inside the zone.
The modest trend is that the greater the outside zone swing rate, the higher the inside zone swing rate. (And Vladimir Guerrerro just swings at absolutely everything.) The ideal player in terms of plate discipline (with all contact abilities aside) would be one who swings at pitches inside the zone and lays off the pitches outside the zone. Dividing the inside zone swing rate by the outside zone swing rate gives us a simple but effective way to measure how good a hitter is at recognizing which pitches to swing at. In 2010, the league average for outside zone swing rate is .274 and the league average for inside zone swing rate is .635; using these numbers, we can get a plate discipline score centered around the league average (a la ERA+ and OPS+).
|Name||Z Swing||OZ Swing||Plate Discipline|
Remember, this has to do only with discipline/pitch recognition. Clearly, hitters that have other skills (raw contact, power, speed) have a lot of worth to a team even if they have poor plate discipline. Brett Gardner is a great example of this. While Brett may take too many pitches, he almost never swings and misses. His .938 contact rate is fifth best in the majors, and on swings in the zone, he makes contact 99.3% of the time, which is best in the majors.
I did a similar post a few months ago; thanks again to Joe Pawlikowski of River Ave. Blues for getting me thinking about this topic.
Data is from Fangraphs.