Expected Weighted On-base Average (xwOBA) is a relatively new statistic that was born with the institution of the Statcast system in Major League Ballparks in 2015. To understand xwOBA, you should get a good grasp of wOBA first.
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The main goal of the advanced statistics that have come from Statcast 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. Now we can take the details of a batted ball and compare it to hundreds and hundreds of similarly struck balls from the past to see what is likely to result from it.
What happens is a batted ball is categorized based on the comparison of its exit velocity and angle to the actual results of all of the similar batted balls from the past. For example, if a ball is hit at 90 miles per hour at an angle of 30 degrees, and batted balls like that have gone for a single 70% of the time in the past, that batted ball will be categorized as an expected single.
xwOBA is the same calculation as wOBA, but it uses the expected totals instead of the actual totals. If a player had 75 singles but only 60 expected singles (due to good luck), you would expect his xwOBA to be lower than his wOBA. This takes a lot of the luck out of the wOBA calculation by restricting the inputs to only things the batter had full control over (strikeouts and walks are factored in before the final xwOBA number comes out as well).
The way to use this for fantasy baseball purposes is for identifying possible buy low and sell high candidates. You will find some players with much higher xwOBA’s than wOBA’s, which could mean they have been very unlucky and you might be able to acquire them for a discount. Conversely, a player with a much higher wOBA than xwOBA may have had a bunch of bloops fall in, or some other element of good luck, so that player should be looked at as a potential sell high.
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Jon Anderson is a featured writer at FantasyPros. For more from Jon, check out his archive and follow him@JonPgh.
