BABIP, also referred to as a player's hit rate, is the rate at which batted balls end up as base hits. There is one caveat with BABIP – it removes home runs from the equation because technically the ball isn't in play on a home run (it never lands in the field of play). Here is the simple formula in play for the measure we call BABIP.
(H-HR)/(AB-K-HR+SF)
A few points to consider when it comes to BABIP.
(1) The major league average for BABIP is traditionally in the .290-300 range.
(2) Players tend to establish their own level of BABIP production over the years. That means if a guy has produced a .275 mark for three seasons and then he suddenly produces a .325 mark, chances are better than even that his BABIP number will regress in the coming season. However, if a player produced a .325 mark year after year and slumps to .275 one year that doesn't always mean he will remain at that lower mark moving forward. Whether good or bad, players tend to establish a fairly consistent pattern over the course of multiple seasons (a rolling three season time frame gives a very solid gauge as to what expectations are for the current/coming campaign).
(3) A player who is performing at a rate that is below his established level can be said to be hitting in a bit of "bad" luck. That's too simplistic of course, but it's a down and dirty way to look at it. A player who is performing at a rate that is above his established level can be said to be hitting in "good" luck. This is also far too simplistic but it suits the purpose of this piece.
(4) An all or nothing slugger like Chris Carter has a large portion of his at-bats result in a home run, a walk or a strikeout (275 pf his 507 plate appearances ended in one of those three events meaning that 54 percent of his trips to the plate ended in a home run, walk or strikeout). Therefore, his BABIP mark could show more fluctuations than most. At the other end of the spectrum, there are swing at everything/contact type hitters like Ben Revere. In his 626 plate appearances last season, he hit two homers with 49 strikeouts and 13 walks leading to just 10 percent of his plate appearances ending in one of those three events. Guys like Revere have more of a stable base to work with the thinking being that 500 plate appearances gives a guy a big enough sample size that things will stabilize versus being randomly effected by a couple of batted balls finding a guys glove when they may not have last year. Regardless, remember that players tend to establish their own baselines.
(5) BABIP isn’t just a useful tool to gauge hitters. Obviously if the major league average is in the .290-300 range, then it is easily identifiable when a pitcher is hurling in “good” or “bad luck” as well (again too simplistic but it suffices for what we're trying to accomplish here). Pitchers tend to establish a baseline for the BABIP mark, though a bit more variance than that posted by hitters can be expected. Still, if a pitcher owns a career BABIP mark of .305 and then one year posts a .240 mark, chances are pretty good that number will regress in the coming year.