what process do you usually use to determine if hitters are under or over-performing? It seems like there are more metrics like FIP, xFIP, LOB%, BABIP, etc. for pitchers, but fewer such stats (maybe BABIP, K or BB rate, and some in-zone out of zone swinging stats among others) for hitters. Any insights would be much appreciated!While there isn't as much focus on these types of metrics for hitters, Fangraphs, Baseball Prospectus, and others have done a pretty good job of breaking down what to look for as far as a hitter that might be underperforming or over performing his base skills. I recommend reading their work on the subject; my version of what they do is scribbling on the back of a napkin.
But since you did ask me to scribble, here are a couple of things that I do peek at from time to time.
BABIP vs. LD%
This is one of the first things I look at as far as whether or not a hitter stinks or is simply hitting into bad luck. As a general rule of thumb, a hitter's "expected" BABIP should equal his LD% plus 1.2. For example, if a hitter's line drive percentage is 20%, then his BABIP "should" be .320. I put "should" in quotes not to incorrectly emphasize a point (as some incorrectly do) but rather to express caution. BABIP isn't this cut and dried. Hitters with a good amount of speed are likely to outperform their "expected" BABIP, while some hitters have a certain "base" level of BABIP that they start with (Tristran Cockroft of ESPN had a great piece on this concept this spring). As a general proposition, though, if you see a hitter in April with a 15% line drive rate and a BABIP of .370, chances are more than good that he's not going to keep it up.
IFFB% and HR/FB%
Besides line drive percentage, the other batted ball metrics I like to look at are infield fly ball rate and home run/fly ball rate. I used to just look at HR/FB rates and assume that a hitter with a high rate was lucky while a hitter with a low rate was perhaps unlucky. However, a hitter with a high HR/FB% rate an a low IFFB% rate is hitting the ball into the outfield a lot when he is hitting a ball into the air and thus more likely to jack one out of the park than a hitter with a high IFFB%. For example, if you're wondering where Alex Gordon's power has gone this year, his IFFB% has jumped from 4.4% in 2011 to 20% in 2012. Curtis Granderson's IFFB% dropped from 13% in 2009 to 7.2% in 2010. While I didn't think he'd become the monster power hitter he became, I did see a HR spike coming.