“Baseball is only a game, a game of inches and a lot of luck,” said the great Bob Feller, a Hall of Fame pitcher cited as one of the top flamethrowers in MLB history. This quote came during World War II, as Feller missed three seasons of his prime to participate in it, and 80 years later, it’s fair to wonder if his claim is still valid. Today, we’ll be analyzing the year-to-year correlation of luck for an MLB player, hoping to conclude whether some players are consistently more lucky than others.
To measure luck, we’ll be using the difference in overall statistics versus expected statistics. Going back to 2015, when Statcast was created, we’ll measure the correlation of the difference in the following statistics from one year to the next: batting average versus expected batting average (xBA), slugging percentage versus expected slugging percentage (xSLG), weighted on-base average versus expected weighted on-base average (xwOBA), and earned run average versus expected earned run average (xERA). These expected statistics are based on the quality of contact, while the overall numbers are the actual outcome for that specific player in a given season.
We’ll start with hitters. Since expected statistics are based on exit velocity and launch angle, some players should be naturally gifted at hitting the ball in the right place, or beating out ground balls, right? Well, not according to our results:
Batting average is known to be a more fluky statistic based on luck, so it’s not surprising that only 3.3% of a player’s luck in that statistic is explained by their luck in the previous season. For all three statistics, however, it’s clear that the coefficients of determination are low enough to suggest that luck isn’t sustainable; there will be some outliers, yet in most cases, it’s clear that hitters who rely on external factors could be in for a rude awakening.
Now, one may argue, there have to be pitchers who can manipulate hitters enough to be lucky. In actuality, though, luck is less sustainable for them than it is for a hitter:
With the exception of ERA, the coefficient of determination between the difference in expected statistic and actual outcome from one year to the next is exactly zero. In other words, how lucky a pitcher is in one season has practically zero effect on how lucky they are in the following season.
So, what should we take away from this information? For hitters, there are definitely some players that are consistently lucky or unlucky, which makes sense. Sprint speed and the use of defensive shifts can certainly affect if a player gets a hit or not, and in the end, those two confounding variables aren’t usually going to change from one year to the next. For your normal hitter, however, teams shouldn’t expect a player who benefitted from a lot of luck to be as successful the following year, especially those who rely on batting average. This could be bad news for players like Fernando Tatis Jr., Tim Anderson, Keston Hiura, Kris Bryant, and Luis Arraez, who each vastly overachieved their expected batting average. Meanwhile, with there being no relationship between a pitcher’s luck from one year to the next, their ability to strike hitters out should theoretically be even more important, especially since we’ve already found that strikeouts are four-times more predictive of success than walks. Therefore, pitchers who struck hitters out and didn’t see ideal results last season, such as Mitch Keller, Corbin Burnes, and Matthew Boyd should naturally post better numbers than they did last season, while the opposite is true for Zach Plesac, Yonny Chirinos, and Jeff Samardzija, who aren’t as proficient in missing bats.
In statistics, the law of averages is defined as “the principle that supposes most future events are likely to balance any past deviation from a presumed average.” This is definitely the case in baseball, and for that reason, judging players by their outcome-based statistics, rather than their expected statistics, is not the right approach when projecting their future performance. If one needs examples of this claim being valid, look no further than recent “breakout” players who relied on luck, such as Rockies pitcher Kyle Freeland, Giants pitcher Dereck Rodriguez, and Braves third baseman Johan Camargo, each of whom saw their numbers dramatically decline from 2018 to 2019. Especially in a shortened seasons where fluke seasons can be expected, it’ll be interesting to see if certain front offices are able to take advantage and sell-high/buy-low on players who deal with the extremes of luck.