Chuck Finder posted over on the Post-Gazette’s PBC blog that Pirates pitching coach Joe Kerrigan has suggested that SP Zach Duke wore down late last season, perhaps due to overwork.  Kerrigan specifically notes August first as the cut off date for where these troubles began, and based on raw ERA, that seems to bare out as Duke posted a 3.26 ERA before that date and a 5.80 ERA after.  I was curious, and wanted to see if the a similar differential existed in his FIP, or if such a large difference was caused, at least aprtially, by things beyond Duke’s control, ie some defensive regression due to 2 key middle infielders being traded.  Lucky for me, the formula to calculate FIP is here (yes I realize that formula isn’t perfect, as true FIP involves a league and season specific factor, and not a generic 3.2 scaling factor, but it will work for my purposes since were talking about one pitcher, in one league, during one season) and baseball-refernce.com makes available game logs with in context summations here.  using those 2 source, I came up with the following.

Pre Aug 1:  146.1 IP, 75 K, 35 BB, 13 HR, 4.05 FIP

Post Aug 1: 66.2 IP, 31 K, 14 BB, 4.85 FIP

So, did Duke’s true performance level get worse late in the season? Absolutely.  But was it as bad as raw ERA would indicate? No.  He wasn’t as good prior to August 1st as the 3.26 ERA would indicate, nor was he as bad after that date as the 5.80 ERA would indicate.  He was somewhere in the middle the whole time.

Note: While I can’t be entirely sure, I have reason to believe that the 2 month sample size post Aug 1 is too small to be statistically significant, and even the 4 month sample size prior may also be a bit too small to truly matter.  I’m publishing it despite being aware of that because the PBC blog post (linked above) peaked my interest enough to look up these numbers and I figured maybe someone else might have the same thoughts I did upon reading that post.