I’m a big believer in the utility of batting average on balls put in play, or BA/BIP, as a means of determining which players are likely to improve or decline over the rest of the season. The theory is that pitchers can only consistently control four things – home runs allowed, walks allowed, strikeouts recorded, and ground ball to flyball ratio. These four statistics show a strong correlation from year to year.
While pitchers can control the number of balls that are put into play (by striking out a lot of guys, or not), what pitchers cannot control is the batting average they allow on balls that are put into play. Similarly, hitters can only control how often they put the ball into play (by not striking out), as well as how many homers, walks, and flyballs vs. groundballs they hit, but not the batting average on those balls they put into play.
This makes sense intuitively, as we often suggest that it was not a pitcher’s fault if ground balls happen to find holes or little flares happen to drop in, just as we sympathize with a hitter who hits laser beams but they just happen to be hit right at a fielder.
Thus using BA/BIP, we can quickly decide which players have been extraordinarily lucky or unlucky with balls falling in or happening to be hit straight at someone. This was how we could tell last year that even though Andre Ethier was leading all major leaguers with a .393 batting average last July, he was not exactly the second coming of Ted Williams, because his batting average on balls put into play was the second best in baseball at the time, which suggested that he was getting extraordinarily luckly with balls dropping in for hits.
Conversely, if a player is doing extremely well or extremely poorly but his BA/BIP is not extra ordinarily high or low, we can assume that he is actually just that good or just that bad.
Looking at some of this year’s performances so far, we can see that Daisuke Matsuzaka has been extremely unlucky. Not only did he come into last night’s game with his team putting up a league low 1.2 runs of support behind him, but he also is in the bottom 10 pitchers in the batting average on balls put in play against him, yielding a .333 average any time an opponent doesn’t strikeout or hit a home run (.290 is about average). It is fortunate that Daisuke gets as many strikeouts as he does or his bad luck so far would have translated into an extremely high ERA.
Similarly, looking at batters, we can see that Gary Sheffield (currently batting .119) is very likely to improve in the near future as he has a second-worst in-the-AL .140 batting average on balls put into play, whereas BJ Upton (.340 avg) has been extremely lucky, with a staggering, major-league-leading BA/BIP of .536, which means his performance is very likely to decline in the near future.