Baseball Savant today released a new leaderboard, dubbed “Rolling Windows.”
On the very surface level, it’s a way to quickly see who has been the best-trending hitter or pitcher over a 50, 100, or 250 PA span. If you’re someone who has stumbled onto this website and doesn’t want to know anything more about advanced statistics, you can safely look at the top 50 PA xwOBA leaderboard and know that Greg Allen has been hitting the ball extremely well recently, compared to how he did in his 50 PA prior to the current window.
Move down to the 250 PA xwOBA leaderboard, and suddenly you can see who has been a consistently hot hitter over roughly half a season worth of at-bats compared to their previous 250 PA. A much more comprehensive view of how well someone is playing compared to 50 games. That can be the end of it if you aren’t interested in the inner workings of what stats the Rolling Windows leaderboard uses.
But if you want to go down the rabbit hole, the Rolling Windows leaderboard, like most things on Baseball Savant, is a nerd’s playground.
Being that this is Baseball Savant, the home of Statcast, it’s not determining these trends using something simple like batting average, or even something more advanced, yet flawed at small sample sizes, like wOBA.
No, it’s going all-in on a stat developed from measurements gathered by Statcast: expected weighted on-base average.
Before getting into xwOBA, it’s good to know about its plucky cousin, wOBA. Essentially it’s a better version of OPS. Instead of slamming on-base percentage and slugging percentage together to form a number, wOBA properly weights the importance of getting on base compared to hitting three triples and being blanked for a week straight. FanGraphs has a great explainer on wOBA that’s worth a read if you’re interested in diving more into it.
Expected wOBA, or xwOBA, uses more advanced measurements from Statcast to determine how well a player should be hitting based on how well they’ve hit the ball and how likely they are to beat out certain types of hits. Specifically, xwOBA utilizes launch angle, exit velocity, and occasionally sprint speed (in the case of a topped or weakly hit ball) to measure what a player’s wOBA should be without the added variable of defense, bad bounces, or the like.
For example, a player consistently hitting the ball over 100 mph with an optimal launch angle might be hitting a lot of balls to waiting outfielders or even line drives into the teeth of the shift. Most every other stat will point to that player struggling without digging any deeper. But not xwOBA. Using those high exit velocities and good launch angles, xwOBA just wants you to know that this mystery player is hitting the ball well, even if the results aren’t there yet.
Expected wOBA looks purely at the quality of contact and speed of the runner on balls that aren’t hit hard. Greg Allen can probably leg out a weakly hit ball better than Carlos Santana, and his xwOBA on a topped ball that has a chance of bouncing weakly to the third baseman should reflect that. Thanks sprint speed! Likewise, if Carlos Santana hits a ball 104.5 mph with a 23-degree launch angle, that’s some good contact, regardless of if it results in an out. That should benefit his expected wOBA.
No stat is a comprehensive number for everything about a player, or for predicting everything about a player, but xwOBA is an excellent tool to find hitters with some bad luck (high xwOBA, low wOBA), good luck (low xwOBA, high wOBA), or just look at how well they’re hitting the ball, or in the case of the Rolling Windows leaderboard, how they’re trending.
Currently, Greg Allen leads the league with a .238 xwOBA delta over his last 50 PA — no batter is trending higher than Allen over two or so weeks’ worth of playing time. Conversely, Carlos Santana has one of the worst changes in xwOBA over his last 50 PA at -.129.
Pulling back to the more telling 250 PA xwOBA leaderboard, Roberto Pérez has the fifth highest change at .081. Carlos Santana isn’t too far behind at .048 and Tyler Naquin is right behind him at .045. You’ve probably noticed without this leaderboard that Tyler Naquin is indeed trending upward, but now you have statistical proof.
For pitchers, Nick Goody has lowered his xwOBA against in his last 50 PA pitched by .196, the third-best trend in baseball. Over on the 250 PA xwOBA pitcher leaderboard, old friend Josh Tomlin has lowered his xwOBA by .089, the third best in the league. This is backed up in more traditional stats by Tomlin’s 3.81 ERA this season, compared to the 6.14 ERA he put up in 2018.
Like almost everything on Baseball Savant, it’s a lot data presented in a nice format. If you haven’t already dug around the Statcast Leaderboards — or their sublime search Statcast Search — do yourself a favor and get lost in the numbers for an hour or seven.