by Daniel Gettinger

From an offensive perspective, players who get on base are good.  Players who hit for power are too. Players who both reach base frequently, and hit for power are really good.

Because a good measure of getting on base is on-base percentage (OBP), and a decent measure of power is slugging percentage (SLG), it has become somewhat mainstream to evaluate offensive performance by combining the two measures into OPS (OBP+SLG).

Unfortunately, adding SLG to OBP, while easy to calculate, is not an entirely accurate measure of offensive performance.  The main reason is OPS treats OBP and SLG as equals that are scaled in the same manner.  In fact, OBP is actually more important than SLG.  Given two players with the same OPS, the one with the higher OBP is better offensively.

Inside the Book Blog recently published an excerpt from PAAPFLY that illustrates the issue:

Continuing my bashing of Bengie Molina, allow me to show you how his terrible OBP can be quite detrimental. Bengie Molina posted a .727 OPS in 2009, which isn’t very good. Ryan Theriot managed to post an even lower OPS of .712 in 2009. He must be the inferior offensive player. Wrong. Molina’s wOBA is actually .308 to Theriot’s .318. Though Theriot slugged 73 points less than Molina, his OBP was 58 points higher, and, wOBA shows us that his 58 OBP points to Molina’s 73 slugging points were actually worth an additional 10 points in wOBA. This is just a quick example and a good way to illustrate just how much Molina’s extraordinary out making skills truly do hurt his team, offensively of course.

From a Padres’ perspective, I liken the Molina/Theriot example to that of Chase Headley and Kevin Kouzmanoff.  This is how Kouzmanoff and Headley stacked up in 2009:

OBP SLG OPS wOBA
Kouzmanoff 0.302 0.420 0.722 0.312
Headley 0.342 0.392 0.734 0.328

Although not a perfect example (because Headley’s OPS was slightly higher than Kouzmanoff’s OPS), we can see that Headley’s wOBA is 5% greater than Kouzmanoff’s wOBA, compared to only a 1.6% difference in OPS. The reason is Headley’s superior on base skills.

OPS is an okay estimate of offensive skill.  But the only advantage it has over a stat like wOBA, which properly weights OBP and OPS, is ease of calculability.  It would be nearly impossible to calculate a player’s wOBA based on the information given by the stadium scoreboard, or the Channel 4 stat-line.  However, outside of convenience, there is no good reason to use OPS.

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