Carleton: Will StatCast cure our defensive metric blues?

From Russell Carleton at Baseball Prospectus on September 23, 2014:

Remember a few weeks ago when Alex Gordon was leading the American League in WAR? No one questions that Gordon is having a(nother) really good season and should rightly get some down-ballot MVP votes, but the best player in the American League? People quickly noticed that a good chunk of Gordon’s WAR came from his defensive ratings, where, at the time, he was picking up roughly two wins worth of value in left field. Gordon’s regarded as a good left fielder, but “good left fielder” is also the “great personality” of fielding aficionados.

It’s been known for a long time that a single year’s defensive ratings, particularly for outfielders, isn’t a reliable indicator of a player’s talent level. It might accurately represent what he did in the past year, to the extent that the data sources that we have available can do that, but it doesn’t tell us what he really is. Commonly, I hear “you need three years’ worth of fielding data to get a reliable sample.” That’s fine if we want to know how good a defender someone really is, but when we are trying to figure out questions like “Who had the best 2014 season among these 15 randomly selected teams?” it means that a good chunk of that value is based on a stat that could just be a mirage.

I often hear people at this point appeal to technology. StatCast, the much-discussed stalking mechanism, erm … tracking technology that will tell us exactly where everyone was at all times on a baseball field will save us. Earlier in the year, MLBAM put up this lovely teaser video of Jason Heyward making a fantastic catch to end a game from July 2013. Wanna watch?


Currently, even the best “advanced” defensive metrics are based on data sources that have a lot of holes. Stringers manually input where a ball landed. They make judgment calls on whether the ball was a line drive or a fly ball. There’s very little data on how long a ball was in the air or how fast it was hit. No one tracks where a fielder was and how far he had to run. The metrics do the best they can with what data are out there (and it’s a heck of a lot better than fielding percentage), but what if we had better data? StatCast is that data set.

Or is it? As we can see from the Heyward video, we can expect to have information on fly ball hang time, fielder positioning (and distance from the eventual landing place), fielder reaction time and foot speed, and the length of the route that he actually took to get to the ball and how efficient that route was. Surely, more refined data will make for a better fielding metric!

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Originally published: September 23, 2014. Last Updated: September 23, 2014.