I harp a lot about BABIP (Batting Average on Balls in Play) and how it's a good indicator if a player is due for a slump or a rebound from year to year, and even within a season. The general guideline is that a player will generally settle within a range of .290-.320 on their BABIP, with the league average being .300 for a hitter and .290 for a pitcher. Now pitchers have much less control over their BABIP than hitters, that is unless they throw a knuckleball or particulary good change-up that is hard to get good contact on, but hitters actually can outperform or underperform that guideline quite significantly thanks to an ability to hit line drives, speed and a few other minor factors. That being said, they don't outperform it by that much. If you look at the 3-year leaderboard for BABIP on Fangraphs, only three players have topped the .360 mark (Jeter, Holliday and Chipper Jones with Ichiro just missing). Now those are some of the best hitters in the game and their career BABIP's are pretty high as well (except for Chipper who has a .328 career BABIP) and it's been shown that players regress more towards their own BABIP levels than the league averages. That all being said, when you see a player hitting anything over .340, you need to start worrying that it will fall back a bit unless they've been able to sustain it for a few seasons. On the flip side, if a guy is suddenly below .280 he's either had a really unlucky season or he's about to leave major league baseball (see Jones, Andruw).