OPS+ and ERA+ are the most well-known home park adjusted stats. They take what the player did overall, then adjust relative to the league average and what the average hitter did in his home ballpark. Parks like Yankee Stadium and Fenway Park generally make hitters look better, while Minute Maid Park and Citi Field make hitters look worse (and pitchers better).
What has always bugged me has been the fact that those stats don't account for the handedness of the player. That usually doesn't matter in symmetric ballparks like Comiskey Park (U.S. Cellular Field) and Skydome (Rogers Centre), but most parks tend to favor either righties or lefties.* Why should Yankee RHB be penalized the same as LHB for playing in a stadium that helps those lefties significantly more than their counterparts?
In 2010, lefties OPS'ed .088 higher in Yankee Stadium than the ML average (FYI, that's about the difference between A-Rod's and Brett Gardner's 2010). RHB also OPS'ed higher, but not nearly to the extent of LHB: .018 to be exact. In percentage terms, that equates to a 12% boost for LHB and 2.5% for RHB. So to neutralize a hitter's OPS for his home ballpark (in this case, Yankee Stadium), we must reduce his OPS by the aforementioned 12% or 2.5%, depending on his handedness. There's one more step though: He only plays half his games at home, so those adjustments should be cut in half, leaving us with 6% and 1.25%.
Let's go to the videotape! (First the actual OPS, then the hOPS (handedness adjusted OPS)) -
Cano: .941, .885
Gardner: .762, .716
Jeter: .710, .701
Rodriguez: .847, .836
As you can see, the batters got a nice boost from Yankee Stadium, and especially the lefties (Jeter is almost a LHB in the way he hits the other way, but I'm not going to tackle that right now).
Now for a pitcher's ballpark: Minute Maid Park (or the park that the Houston Astros play in for all the corporate-name challenged fans, of which I'm one).
It decreased LHB OPS by .011, but RHB suffered mightily: The park reduced their OPS by .056. That equates to 1.5% and 8.4%, respectively (or .75% and 4.2% when dividing in half). This will make Houston's batters look better.
Carlos Lee: .708, .738
Hunter Pence: .786, .819
Michael Bourn: .686, .691
You'll notice Lee goes from roughly the Derek Jeter-level to significantly higher whilealmost reaches A-Rod status. LHB, however, only got a slight boost, as can be seen in Bourn's hOPS.
Now for pitchers. This is where Yankee pitchers will look better and Houston hurlers will take a hit. The general consensus that southpaws fare better at Yankee Stadium is, in fact, true. LHP conceded a .730 OPS Against while RHP conceded an .807 OPS Against (FYI, the rough difference between Marcus Thames' and Cano's 2010). Compared to the ML average (.727 OPSA LHP, .729 OPSA RHP), southpaws had a much easier time in The House That George Built. That amounts to a -.4% drop for LHP and an 11% increase for RHP (ergo, -.2% and 5.5%).
The results -
Sabathia: .656, .657
Pettitte: .703, .704
Hughes: .702, .663
Burnett: .824, .779
As you can see, The Stadium is pretty fair to lefties but hurts righties a lot, the reason the Yanks put a premium on southpaws. Hughes and Burnett, in average ballparks, would have much easier times of it. In fact, Hughes hOPS Against is almost identical to Sabathia's; he got killed at home - if you'll remember, he allowed 20 of his 25 homers in the Bronx.
Now back to Houston, where pitchers could "get away with" a lot more than blue-pinstriped pitchers. If you don't mind, I'll skip the equations this time and go right to the results -
Brett Myers: .683, .703
Bud Norris: .758, .777
Wandy Rodriguez: .700, .721
Minute Maid Park is more even than Yankee Stadium regarding the handedness of pitchers, with only a .006 OPSA difference between RHP and LHP, but the overall theme is that hitters have a tougher time in Houston. As shown above,, who posted an unadjusted OPS below that of , got a big boost from his ballpark and would fare significantly worse than Hughes in a neutral park.
Hopefully this will inspire a website like Beyond the Box Score or Fan Graphs to study the issue in more depth. There's a lot here, and a lot than can be gleamed about the true value of players and the inner workings of the game.
* You may have noticed that I decided not to tackle switch-hitters in this study. OPS+ might seem like the best option considering they bat from both sides of the plate - and therefore even out the handedness differences - but they generally get a lot more PA against RHP than LHP (e.g., Mark Teixeira had 486 PA against RHP and 223 against LHP last year). That would require weighting, and a lot more work than I have time for.