Every year you see pitchers come out of nowhere to have phenomenal first halves of the season, or great ones surprisingly struggle. And for every Roger Clemens, there's a Jack Armstrong. The trick is to figure out when a pitcher's breakout is for real, or just a mirage.
Given that, here's some pitchers that could very easily slide one way or the other. Two of the biggest indicators that I usually look at are hit % (the percentage of balls in play that fall for hits), and strand % (the percentage of baserunners that fail to score). Generally, those numbers shouldn't slide too much past 30% and 70% respectively - if they do, you're looking at a potential correction. Hardball Times and Baseball Prospectus go way deeper into these, and about a million other statistical measures.
Justin Duchscherer (OAK) - 10-5, 1.78 ERA - 21 walks in 101 IP is nothing to sneeze at. But, you look at that 5.6 K/9 IP, and you look at that 21.6 H%, and you've got to figure that eventually, those balls that are hitting the bats will eventually fall for hits.
Gavin Floyd (CHW) - 10-5, 3.63 ERA - He might be the most likely of any MLB pitcher to regress. Hit % of 22.7, S% of 76.1, and a strikeout/walk ratio barely over 1.5. Plus, he's got no real history of success in the majors. Red flags everywhere with this kid.
Armando Galaragga (DET) - 7-3, 3.27 ERA - He kind of came out of nowhere to become the Tigers' stopper. But there's just nothing there to support these numbers. He's young, so the control could improve; right now he's just an average control pitcher with a lucky 76.5 S%.
Joe Saunders (LAA) - 12-5, 3.07 ERA , Scott Olsen (FLA) - 5-4, 3.77 ERA , Greg Smith (OAK) - 5-7, 3.43 ERA - All three have H% around 25, S% around 75, none have great control, and all three make Galaragga look like a flame thrower. All three are easily candidates to fall apart. Go ahead and throw Aaron Laffey (5-5, 3.45 ERA) in there as well.
And for some possible returns to form:
Bronson Arroyo (CIN) - 7-7, 5.97 ERA & Aaron Harang (CIN) - 3-11, 4.76 ERA - Inflated hit rates, together again! Both of them are throwing strikes (8.0 K/9 IP), both have very respectable control, and both have H% over 33. Good chance for a turnaround. There's one caveat though; the fact that these guys have high hit rates may be because the Reds defense stinks, so there may not be as big of a correction as expected.
Roy Oswalt (HOU) - 7-8, 4.56 ERA - He's not striking out batters at a huge rate, but he's still got a 3.4 K/BB ratio, and his hit rate is a little high. As consistent as he's been throughout his career, he could easily post big numbers in the 2nd half.
Javier Vazquez (CHW) - 7-7, 4.61 ERA - Javy's been throwing some serious heat this year (117 K's in 121 IP), but he's got an unlucky hit rate close to 34. He's one of those guys that just never goes away, so expect him to improve considerably.
Fantasy Baseball, MLB