One of the things we are always trying to do in fantasy baseball is identify who may break out or change their current course. For pitchers, we look at gaps between ERA and FIP, or more accurately, DRA. Hitting has not always been as easy. For years, we would look at high or low BABIP’s or home run to flyball ratio and try to predict what would happen in the coming week.
The good news around that is the tools we have to work on such predictions are becoming more accurate, thanks to the great work around StatCast and the people doing the research around what is being measured. Weighted On-Base Average (wOBA) recognizes that not all hits are created equal whereas batting average does. Batting average does not care how many bases the batted ball resulted in, it just cares that X number of hits in Y number of plate appearances equals an average. It is the simplest way for fans at a game or looking at a box score to get a feel for how a hitter is doing at the plate. It is far from accurate and not a decent measure for offensive contribution, but it will likely live on for awhile because it is easy to explain. wOBA is not so easy. After all, look at the formula for computing wOBA:
Imagine explaining that one to your mom and dad the next time you go to the ballpark? As the formula shows, not all hits are created equal and wOBA combines all the different aspects of hitting into a single metric weighting each of them proportionately to their actual run value. In short, wOBA is a more accurate measure of offensive productivity than batting average, on-base percentage or slugging percentage. For a longer definition of wOBA, please see the Fangraphs library.
wOBA alone cannot help you predict what a batter will do moving forward, but its cousin xwOBA can help. The x stands for expected and is defined by StatCast as:
Expected weighted on-base average (xwOBA) is formulated using exit velocity and launch angle, two metrics measured by Statcast.
In the same way that each batted ball is assigned a Hit Probability, every batted ball has been given a single, double, triple and home run probability based on the results of comparable batted balls -- in terms of exit velocity and launch angle -- since Statcast was implemented Major League wide in 2015.
All hit types are valued in the same fashion for xwOBA as they are in the formula for standard wOBA: (unintentional BB factor x unintentional BB + HBP factor x HBP + 1B factor x 1B + 2B factor x 2B + 3B factor x 3B + HR factor x HR)/(AB + unintentional BB + SF + HBP), where "factor" indicates the adjusted run expectancy of a batting event in the context of the season as a whole.
Knowing the expected outcomes of each individual batted ball from a particular player over the course of a season -- with a player's real-world data used for factors such as walks, strikeouts and times hit by a pitch -- allows for the formation of said player's xwOBA based on the quality of contact, instead of the actual outcomes. Likewise, this exercise can be done for pitchers to get their expected xwOBA against.
Why is it useful? Because it removes defense from the equation. The original beliefs of BABIP stated that hitters could only control so much of their batted ball outcomes but we have now come to realize that batters do have more control by determining how hard to swing and at what angle. Once the ball leaves the bat, they give up control but the events leading up to the moment of contact are completely in the batter’s control. Take this play from April where Josh Reddick robs a would-be home run from Jason Kipnis: