Batting average on balls in play
In baseball statistics, Batting average on balls in play (abbreviated BABIP) measures how many of a batter’s balls in play go for hits, or how many balls in play against a pitcher go for hits, excluding homeruns. BABIP is commonly used as a red flag in sabermetric analysis, as a consistently high or low BABIP is hard to maintain - much more so for pitchers than hitters. Therefore, BABIP can be used to spot fluky seasons by pitchers, as with other statistical measures; those pitchers whose BABIPs are extremely high can often be expected to improve in the following season, and those pitchers whose BABIPs are extremely low can often be expected to regress in the following season.
A normal BABIP is around .300, though the baseline regression varies depending on number of factors including the quality of the team's defense (e.g. a team with an exceptionally bad defense might yield a BABIP as high as .315) and the pitching tendencies of the pitcher (for instance, whether he is a groundball or flyball pitcher). While a pitcher's BABIP may go up and down in an individual season, there are distinct differences between pitchers' career averages.
The equation for BABIP is: