A few years ago I wrote an article about Matt Cain‘s incredibly unlucky 7-16 win-loss record in 2007. Cain’s example is one of many showing that win-loss records can be affected by numerous factors outside the pitcher’s control, and that we shouldn’t use Wins to gauge pitchers’ abilities.
ERA is another stat influenced by factors outside the pitchers control. A pitcher’s home ballpark, team defense, BABIP, LOB%, and homers allowed all factor into his ERA, yet much of the variability from those stats has little to do with the pitcher himself. Thus, ERA can fluctuate greatly above and below a pitcher’s true ability, making it a poor estimator of that true ability. Translation: Don’t use it.
While Cain’s 57-62 win-loss record is hardly representative of his excellent 3.45 ERA, his 3.45 ERA is hardly representative of his apparent skill set. Perhaps karma is at work. Cain’s peripheral pitching stats–strikeout rates, walk rates and groundball rates–put him in a tier of pitchers who average about a 4.4 ERA, not the sparkling 3.45 career number he has posted. The 4.4 figure comes from Fangraph’s xFIP, a stat that predicts ERA based only on the peripheral stats listed above.
So, is Matt Cain an exception to the rule, an outlier if you will, or is he a random spike ready to come back to earth any season now? That question is impossible to answer for sure, but I can make a guess.
I looked at the 57 pitchers who have thrown enough innings in the last 3 seasons to make Fangraph’s “qualified innings” cut. I focused on the difference between each pitcher’s xFIP and ERA over that span. In other words, I want to know by how much each pitcher’s ERA is under-performing or outperforming his predicted ERA (xFIP). Though Cain has definitely outperformed his xFIP–recording a much lower ERA in the last 3 seasons than xFIP would predict–he’s not in a league of his own. Right up there with him are standout pitchers Johan Santana, Felix Hernandez and Adam Wainwright. It’s tempting, then, to argue that Cain is in a group of overachievers, all of whom have that special something that enables them to beat the averages. Not so fast, of those 57 pitchers, the bottom four (“underachievers”) go like this: James Shields, Javier Vazquez, Ricky Nolasco and Josh Beckett, all fantastic pitchers themselves. This makes it hard to argue that a certain type of pitcher is able to break the xFIP mold.
Then I made a histogram of the xFIP/ERA differences. It’s shape, seen below, is not surprisingly that of the bell curve, or normal distribution. The fact that these pitchers fall into a nice normal curve doesn’t prove anything for sure, but indicates that our measurement of over and under performing has an extremely common distribution, and could be almost completely random. Using the definition of an outlier, none of the overachieving pitchers (Cain, et. al.) were actually outliers. The only two outliers were Nolasco and Beckett on the other side.
Outperforming one’s xFIP may be an actual ability that is itself distributed normally, but I doubt that. Pitchers who record significantly lower ERAs than xFIPs almost invariably have better LOB%s, BABIPs and HR/FB rates. In my own research I have been unable to find a pattern the types of pitchers who perform well in these categories. As seen in this data set, good pitchers were on both ends of the “luck” spectrum, and that tends to be the general case.
There’s no evidence to suggest that Cain is anything special, and I believe he is just a random spike waiting to regress. AT&T is a good ballpark for him, and the Giants defense has been well above average, so I don’t expect his ERA to jump way up over 4.00. But it wouldn’t surprise me to see something close to 4.00 next year.