Ever since the signing of Mike Pelfrey, I've asked myself: "How did he ever have ANY success?". By "success" I'm referring to his 2008 and 2010 campaigns. Granted, he's inconsistent, but the basic results of his 2008 and 2010 season (ERA, FIP, WAR,) seem to show that he's a decent #3- type starter, maybe even #2. Even when we look at his "luck stats" (BABIP, LOB%) from those seasons we can see that they are around the league average. Yet if we take away all of the stats previously mentioned, we see that Mike Pelfrey is only a #4-#5 starter. Let's take a look: 2008:WHIP 1.36, K/BB 1.72, SIERA 4.61, xFIP 4.45 2010: WHIP 1.38, K/BB 1.66, SIERA 4.61, xFIP 4.31
One thing that Pelfrey is good at is getting guys to hit in ground balls (like every other Twins Pitcher):2007 (48.3 GB%) 2008 (49.6) 2009 (51.3) 2010 (47.8) 2011 (45.6). 2008 was his second highest GB%, and 2010 was his 5th highest. So for the most part, it's not the ground balls that are affecting him, since 2009 was the year of his highest GB% yet it was also the year of his highest ERA as a qualifying pitcher.
So after looking through statistics on none other than FanGraphs, I have discovered a correlation between his ERA/FIP/WAR and his HR/FB rate. Take a look:2007 (8.8 HR/FB, 5.57 ERA, 5.06 FIP, 0.3 WAR) 2008 (6.3, 3.72, 3.96,3.0) 2009 (9.5, 5.03, 4.39, 1.6) 2010 (5.7, 3.66, 3.82, 2.8) 2011 (9.1, 4.74, 4.47, 0.7) Notice that 2008 and 2010 were the years of his lowest HR/FB rates, and he had the most success in those two years, on the surface anyways.
While this type of correlation is fairly common among pitchers (and Pelfrey's HR/FB rates are average at their worst), so far it is the only real correlation that I have found between Pelfrey's success and what dictates his inconsistency. Overall, Citi Field is more of a pitcher's park than Target Field (based off of Baseball-Reference's Park Factor Rating), so perhaps we will see higher HR/FB rates with Pelfrey, and therefore less of a chance of him achieving the results of his 2008 and 2010 seasons.
Statistics used from FanGraphs and Baseball Reference