What does it mean when a stat has the word “Expected” in its name?
Stats such as Expected Batting Average (xBA) and Expected Slugging Percentage (xSLG) are considered to be more indicative of a player’s true skill level than their more traditionally used counterparts.
Most statisticians believe that while a hitter or pitcher has the ability to influence the exit velocity and launch angle of a batted ball, the player has little to no control over what happens to the ball once it’s put in play. Thus, “expected” stats are used to determine how successful a player is when defense is removed from the equation.
Take Expected Batting Average (xBA), for example. To calculate this metric, Statcast first assigns an xBA for each batted ball based on how often comparable balls have fallen for base hits. Strikeouts are later added into the equation.
What is meant by “comparable” batted balls?
Statcast compares each batted ball to previous batted ball events based on exit velocity, launch angle, and in some cases, Sprint Speed. So if a ball is assigned an xBA of .500, it means that batted balls with that combination of exit velocity and launch angle have fallen for hits fifty percent of the time since Statcast was first implemented.
What is the purpose of these stats?
Like BABIP or FIP, expected stats attempt to isolate factors that a player can control. Expected stats reflect a fundamental truth of baseball, which is that a player’s numbers in a given stretch are not always representative of how well he is hitting or throwing the ball.
A hitter might post a .220 average over a ten-game span, but if his xBA is .430, one can surmise that he is likely the victim of some poor luck. Ultimately, expected stats are useful because they allow us to make assessments based on a player’s quality of contact.
Case Example: Marcell Ozuna
Marcell Ozuna put up a respectable but somewhat underwhelming 2019 season with the Cardinals. The outfielder hit .241 with 29 home runs and a 107 OPS+ over 130 games for St. Louis. However, while his overall totals were unimpressive, his final statline was not indicative of his true talent level.
Ozuna’s hard-hit rate for the 2019 season ranked in the 96th percentile for all major league players. Consequently, his expected metrics were much higher than his actual stats. For instance, his xSLG was .548 while his actual slugging percentage was .472. A statistician could view the discrepancy between Ozuna’s expected stats and his actual stats as a sign that he could easily bounce back next season.
