In case you missed it, earlier today Jon Heyman tweeted this:
i am not a hater of WAR stat, but if someone can explain to me how starling marte & bryce harper are both 1.7, please do— Jon Heyman (@JonHeymanCBS) April 29, 2013
A lot of discussion and arguing ensued, and to be honest, I wasn't very happy with the way in which some people went about responding to Heyman. But that's not the topic for this post. Let's talk about the core of the tweet: the reliability of WAR and defensive metrics.
Dave Cameron did just that this afternoon, looking at the relationship between winning percentage as calculated by WAR and actual winning percentage, both with and without defensive metrics.
As it turns out, when looking at team-wide WAR, defensive metrics like UZR and DRS do strengthen the correlation between WAR and winning. However, as some commenters pointed out, that still doesn't mean that defensive metrics are useful on an individual level. I tend to agree. But I have a different issue with Heyman's tweet, as I expressed in the comments to Cameron's post.
My issue with Heyman’s tweet is that he’s coming from a position of assuming that Harper has been better than Marte, and then criticizing WAR because it disagrees. Sure, UZR and DRS are pretty close to useless on an individual level after one month. But their values are certainly possible. That is, it’s very conceivable that Marte’s defense has been good enough to make up the ground between him and Harper offensively.
The problem is that we have no idea if that’s true or not. But it could be true, and for Heyman to just assume that it isn’t is incorrect just as assuming that it is true is incorrect.
Here’s another way to think about it. So far this season, Harper has about 14 runs from offense + baserunning and Marte has about 7. Say we don’t know the rest of their value, but we know it’s some random number between -5 and 5 runs.
Now, the odds are not in Marte’s favor. If we have to choose one, we’ll say that Harper is better. But we shouldn’t have to choose one. Because for all we know, Marte’s defense – that random number of runs – makes up the gap between the two offensively. If Marte has +3 runs and Harper has -4, or Marte has +5 and Harper has -2, the difference is eliminated.
Those situations are unlikely, but possible. We can’t come from a position of assuming that the two are equal defensively just because we have no way of knowing their defensive value. We can say that Harper is likely better, but we still need to admit to the fact that he might not be.