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Sabermetrics Glossary: Expected Slugging Percentage (xSLG)

Sabermetrics Glossary: Expected Slugging Percentage (xSLG)

The main goal of the advanced statistics that have come out of the Statcast revolution in baseball is to take luck out of the equation as much as possible. The way to do this with hitters is to drill every event down to only the things they have control over. A hitter has full control over making contact with the ball, but after the ball is put in play, the outcome of the at-bat is influenced by other factors (ballpark factors and the defense). Since Statcast tracking was instituted in all Major League Ballparks in 2015, every pitched and batted ball has been tracked closely, which has resulted in a mountain of data to study. This has led to statistics like Expected Slugging Percentage (xSLG).

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First, we should clarify what ordinary slugging percentage is. Slugging Percentage is simply a batting average-like metric that gives added weight to extra-base hits. A double is worth twice as much as a single, a triple three times as much, and a home run four times as much. A home run and a single look the same in terms of batting average, but not slugging percentage (one-for-one with a single would be a 1.00 slugging percentage, one-for-one with a homer would be a 4.00 slugging percentage). The simplest calculation for this is Total Bases divided by At-Bats.

Since we have several years of data about every batted ball, we can now take an individual batted ball and compare it to thousands of other batted balls that were hit with similar velocities and at similar angles. If you are looking at a batted ball that was hit at 103 miles per hour at a 40-degree angle, you could look to the past data and see that batted balls of that type have gone for home runs a high percentage of the time, meaning a home run would be the highest probability of being the true outcome. You would then categorize that batted ball as an expected home run, which would make the expected slugging percentage for that batted ball 4.00. If you are looking at a batted ball that has gone for an out 70% of the time in the past, you would categorize that as an out, or a zero slugging percentage.

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