Pitcher Win Value Correlations

Now that we’ve worked through the logistics of the pitcher win value formula that we recently added here on FanGraphs, I figured it was time to answer the important question – how predictive are they? Since pitchers have historically been evaluated by ERA, the belief has been that they are wildly inconsistent from year to year. However, we know that ERA includes a bunch of non-pitcher variables, and since we’re using FIP, we shouldn’t have to worry about the variation in those external forces.

So, how well does a pitcher’s Win Value correlate from one year to the next? Better than I expected, honestly. I took all pitchers with at least 10 IP in 2004/2005, 2005/2006, 2006/2007, and 2007/2008, and found the following year to year correlations:

2004 to 2005: .62
2005 to 2006: .69
2006 to 2007: .67
2007 to 2008: .55

That’s not bad at all. ERA, for example, has a year to year correlation to itself of around .4. Clearly, the inputs of FIP are more stable than the inputs of ERA, but we knew that already. However, since FIP is a rate stat and Win Value is a counting stat, I’m a bit surprised at how well the win values hold up, since it requires a combination of similar performance and playing time.

As time goes on, I’m sure we’ll improve the formula and push the year to year correlations higher, but as it stands, Pitcher Win Values 1.0 do a pretty nice job of predicting pitcher value from one year to the next.