Friday, August 20, 2010

Plate Discipline: The Ramifications of Swinging Outside the Zone

In my last article on Pablo Sandoval, I criticized his lack of discipline at the plate, citing his propensity to swing at pitches outside of the strike zone (O-Swing%). It sparked my curiosity and made me wonder what relationships exist between O-Swing% and various offensive performance metrics. My hypothesis going in was that there would be a strong inverse correlation between O-Swing% and things like wOBA, OBP, ISO, and SLG.

(Note, study performed using the 162 qualified hitters at this point in 2010).

What I found was that wOBA and O-Swing% have a correlation of -.19. This was a smaller relationship than I was expecting, but it makes sense. The relationship between a player’s O-Swing% and BB% is very strong (-.64 correlation), but walk rate is also heavily influenced by a player’s ISO (.38 correlation). The importance of the player’s ISO in determining walk rate is tied into the ‘fear factor’ involved in pitching to a hitter with more power.

The relationship between O-Swing% and on-base percentage was pretty strong (-.42 correlation). This is surely due to the increased walk rate patient hitters possess, as the relationship with batting average was essentially non-existent (.03 correlation). The same goes for SLG, ISO (.05, .04 correlation). Patient hitters possessed better BABIPs, but not by enough to draw much from (-.15 correlation).

Now as any good amateur economist/sabermatician knows, correlation does not necessarily imply causation. Just because O-Swing% doesn’t correlate with ISO, SLG, and BABIP doesn’t mean that it’s irrelevant in regards to those statistics. A critical factor here is selection bias. Vladimir Guerrero has consistently been one of the top players in baseball in O-Swing%. Vlad has been able to carve out a place as one of the games better players due to his contact and power skills. Both Guerrero and injured Braves third baseman Chipper Jones have produced .358 wOBAs this season. Guerrero’s average is 34 points higher, and he’s slugging 65 points higher than Jones. However, Chipper’s O-Swing rate is just 21.2% and he’s walking in 16% of his plate appearances, while Vlad comes in at a league leading 46.5% O-Swing rate, causing him to walk in just 6.3% of his plate appearances (despite hitting with good power). The lesson here is that an aggressive player can still be productive, he just has to provide value in other ways to equal the production of a more patient hitter.

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