Batting Average on Balls In Play = (Hits – Home Runs) / (At Bats – Strikeouts – Home Runs + Sacrifice Flies)

The purpose of Batting Average on Balls In Play, BABIP, is to add a luck factor which can be used when evaluating players previous production.  BABIP is a ratio of plate appearances to the numbers of balls put into play.  The result is a ratio similar to Batting Average, however it is measured on a scale which removes home runs and strikeouts.  By removing Home Runs and Strikeouts, BABIP measures the numbers of times which a hit fell when a defender had the opportunity to make a play on it.  If we agree that defenders will get to balls at the same rate over the long run, then BABIP shows us how lucky or unlucky a player is based on their deviation from the mean.

When evaluating pitching, BABIP can be used to explain other statistics like standard 5×5 stats ERA and WHIP.  A high BABIP is generally a sign that a pitcher will improve upon their previous production.  The higher BABIP means that more hits are falling which have a negative impact on ERA and WHIP.  But we expect BABIP to be around .300 and in-order for that to take place, we expect a regression to bring the average to the mean.  So to get to .300, there will be a period of low BABIP to account for the higher BABIP in the past.

Lets look at some examples

How bad was Mike Pelfrey in July?  Well to go along with his 10.01 ERA, Pelfrey put up a insane BABIP of .482!!!  Seriously, half of the balls which were put into play fell for base hits.  There is no way that a rate like that could ever keep going.  Even with his amazing start, his BABIP is .342 for the year, so we should expect improvement to finish the season off.  But keep in mind that Pelfrey has a career BABIP higher than .300, so it may not be a serious improvement.