It seems that sabermetricians today take linear weights for granted. Before the days of wOBA and wRC+, OPS and OPS+ ruled the offensive landscape. That was until people realized that the slugging percentage metric was inherently flawed: the weights associated with it do not actually take into account the inherent run value in each offensive event. That’s why in these new metrics, wOBA and wRC+, the value of each event is taken into consideration with proper weighting to assure that it most accurately assesses a player’s offensive abilities. But, one statistic is still a remnant of this era–ISO (Isolated Power). ISO is calculated rather simply–it’s merely one’s Slugging Percentage minus one’s Batting Average. The equation is as such:
This statistic has the same issues that plague OPS and Slugging Percentage. In Isolated Power, a batter with 3 doubles and 0 home runs has the same isolated power of someone who has 0 doubles and 1 home run in the same number of at bats. As we already know, that’s not the case. The point of Isolated Power should be to calculate the value of a player’s power, and should not give undue weight to home runs and triples. Here, I would like to present an improvement on ISO, in hopes that it can more accurately portray the value of a hitter’s power.
The first obvious change, like I said before, is for linear weights. As well, this new measure would also include Plate Appearances as it does with wOBA. The new equation would look like this instead:
I would call this new statistic wISO, but unfortunately it’s not fully weighted just yet. That’s because this also does not take into consideration Park Factors, which can greatly affect one’s perceived power. To account for this, one needs to find an adjusted amount of doubles, triples, and home runs. Using the instructions at Baseball Think Factory, one can derive a2B, a3B, and aHR using Park Factors courtesy of Fangraphs.
Now it’s finally a fully weighted and adjusted wISO. The final equation looks like this:
What’s a good example of this new metric in action? Chris Davis led the American League in Isolated Power in 2013 at .348, with Miguel Cabrera and Brandon Moss trailing behind him at .288 and .266, respectively. Isolated Power alone would suggest that Chris Davis is significantly more powerful than Cabrera and Moss, by a long shot. What does wISO say though? It tells a much different story. After using the aforementioned equation, their wISO’s respectively come out as .215, .178, and .191. In fact, Davis’ power is not that much better than Moss and Cabrera. Instead of a 20% difference, it is more like a 12% difference between #1 and #2. And more significantly, the #2 in wISO is actually Brandon Moss and not Miguel Cabrera. This is because due to the Orioles park factor, Davis’ aHR was actually 48 and not 53. And Moss, who played in O.co, gets the benefit of a favorable park factor for the Athletics, meaning that his home run total should have been 32 instead of 30. Those are big differences over the course of one season, so one can just imagine how skewed one’s power can be over the course of a career.
The practical purposes of wISO can be easily seen. Someone like Brandon Moss’ credentials, power-wise, can easily be dismissed due to O.co, but with wISO he can be appreciated as the true power hitter he is. This new measure is definitely an improvement, and I’m hoping that it opens the door for others to take a closer look at examining the nature of Isolated Power.
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