I all ready posted these numbers in the comments of another thread, but it generated enough discussion that I thought I should make a full post out of it. I'll give a quick description of each of these stats, but for more info, you can go to The Hardball Times.
ERA - You all know this one. Over the course of a year, this is a fairly good indication of how a pitcher has pitched. For small sample sizes (like 5 starts!) luck and other things can skew this number and make pitchers who have been throwing well look poor and vise versa.
FIP - "Fielding Independent Pitching" This is the expected ERA of a pitcher based on three things - Strikeouts, Walks, and Homeruns. Besides being "Fielding independent", it is also very luck independent. It assumes that all balls in play will become hits at a league average rate, and that you will pitch the same with runners on base as without.
xFIP - "expected Fielding Independent Pitching" This takes the idea of FIP a step further. It uses the idea that most pitchers will usually give up a certain number of homeruns per fly ball given up. Essentially it takes the strikeouts, walks, and flyball rate and gives you the expected ERA based on these stats alone.
For small sample sizes, FIP and especially xFIP will give you a better indication of how a pitcher has thrown thus far, because the factors not used in their formulas (Batting average on Balls in Play, Rate of stranding runners, and HR per flyball given up) will tend to regress to the league mean as the season progresses. These statistics aren't perfect, but they give a better prediction of future play than ERA does.
Here is a breakdown of our 5 starting pitchers based on these three statistics.
We're all pretty familiar with the ERAs. Perkins hasn't given up many runs, Baker has given up a ton. But look at the FIPs. Perkins still looks great, Blackburn very good, and Baker still way up there. The xFIPs are what looks strange. All 5 pitchers come out to roughly the same place. Slowey at about league average, Blackburn about half a run higher, and the rest in between.
Why is this? It comes down to the fact that Perkins and Blackburn have each yielded just one homerun thus far. This will not continue. Pitchers just do not keep the ball in the ballpark while giving up as many homeruns as Perkins. Perkins' xFIP is almost exactly where it was last year, so expect a repeat of 2008 from him as the year progresses.
Blackburn has somehow limited runs this year with an absolutely abysmal strikeout rate - 3.45 strikeouts per 9 innings. For reference, Livan Hernandez's was 3.3 with the Twins last year. I'm sure we all remember how he was able to get hitters out for a while, and then completely fell apart and ended up with a 5.48 ERA before departing. He will not continue this success unless he finds a way to strike out more batters.
The most interesting case is Scott Baker. He has allowed 3.4(!) homeruns per start this year. This will not continue. He has been, and will always be homer prone as long as he gives up as many flyballs as he does, but a rate like that is just not sustainable, and will regress hard toward's the league mean. He hasn't pitched as well as last year, but it hasn't been nearly as bad as his ERA suggests. Expect Baker's ERA to drop rapidly over the next month.
Overall, I'm not sure if I'm more or less optimistic after looking at these numbers. While it looks like Baker will rebound, this will be counteracted by Blackburn and Perkins going the other direction. Overall, our pitchers need to start striking people out or we're in for a long season.