Gennaro: Quantifying the quality of opponents

From SABR President Vince Gennaro at Diamond Dollars on October 8, 2013:

Many baseball analysts strive to create “context neutral” stats, so they can compare players stats, while minimizing biases. A player’s traditional stat line–batting average, home runs, OBP, slugging percentage, OPS, etc.–are a product of more than the player’s talent level and luck. For example, they are also impacted by the handedness of the pitchers they face and the ballpark in which they play . These factors can be accounted for with evaluating platoon splits or park-adjusted stats. Perhaps because it is more difficult to measure, advanced stats are seldom adjusted for the quality of opponents. Adjusting for the quality of opponents is particularly important when interpret starting pitcher stats, since we deal with a small sample size of 25 to 35 starts per year, with a start every 5th day, regardless of who is on the schedule. Since talent tends to be clustered in certain Divisions, the unbalanced schedule produces a skewed distribution of opponents. A pitcher in the AL East is likely to be playing a different game than one in the NL East.

I use a simple measure to provide capture the quality of opponents a starting pitcher has encountered over the course of a season–the OPS of the offenses he has faced. More specifically, the OPS of the teams he has faced, against the same handed pitchers. In other words, for David Price, I take the opponents for each of his 27 starts for 2013 and use the OPS against LHP as my measure. If a RHP, faced the same teams, their opposition would be categorized based on their OPS against RHP. For example, Texas mashes LHPs to the tune of a .798 OPS, while they maintain a modest .705 OPS against RHP. On the other hand, the Cardinals hit RHP at a .753 clip, while batting only .675 against lefties. So, it’s not enough to say a pitcher faced the Cardinals or Rangers. It’s also important to distinguish his handedness.

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Originally published: October 8, 2013. Last Updated: October 8, 2013.