The Metric of Baseball Luck
In baseball, when a batter hits a line drive directly into the center fielder's glove, it is an out. When a batter weakly breaks their bat, and the ball slowly trickles onto the grass where nobody can catch it, it is a hit. The batter did everything right in the first scenario and everything wrong in the second, yet the statistics reward the failure.
Batting Average on Balls In Play (BABIP) was created to measure this exact phenomenon. It mathematically quantifies luck.
The True Average
BABIP completely removes Home Runs and Strikeouts from the equation, because neither of those events requires a fielder to make a play. It only measures what happens when the ball is actually put into the field of play.
The Formula
The numerator is all hits (excluding Home Runs). The denominator is all plate appearances where the ball was put in play.
BABIP = (Hits - HR) / (At Bats - Strikeouts - HR + SF)
Reading the Tea Leaves
Across over a century of Major League Baseball history, the league-average BABIP always settles directly around .300. This means that 30% of balls hit into the field will become hits.
- If a batter has a .380 BABIP, they are not suddenly a god. They are getting insanely lucky, finding holes in the defense. They will regress to the mean.
- If a batter is in a massive slump with a .210 BABIP, they should not change their swing. They are just hitting into bad luck, and the math guarantees their hits will eventually start falling in.