Evaluating Defense
If you’ve been hanging around the blog for any length of time, you’ve probably come to realize that we like numbers. They give us a better way to evaluate what we think we saw, and they compensate for our internal bias’. Since a lot of baseball is essentially a set of isolated individual plays, it’s fairly easy to evaluate a player’s value to the team through their statistics, if you know which ones to use.
However, defensive evaluations have always been elusive to the statistical community. The numbers that were recorded, such as fielding percentage, were basically useless information, more misleading than anything else. For years, the players who have made the most memorable plays have been regarded as the elite defensive players simply because we’ve had no real objective standard of how to evaluate defense.
In the past 3-4 years, however, we’ve seen significant steps forward in the realm of defensive statistics. People interested in understanding the game better have begun purchasing play-by-play data that gives them far more information than we’ve had available previously, and have used that specific information to create systems that do a much better job of figuring out just how much value a player’s defense adds to his team. However, the age of defensive statistical analysis is still in its infancy, and as such, there is not a consensus system that is correct, or established as the industry standard. There are several systems built on solid theories that evaluate different parts of defensive prowess, and sometimes, these systems give widely contradictory results. So, what do we do then, if two systems, both well designed, can’t agree?
At this point, my preference is to take a prism perspective. All of the systems have strengths, and all have flaws. So I’d rather not take any of them at face value, but instead develop a general idea of a player’s abilities based upon as much good input as I can get. So, since it’s been requested and there’s nothing going on in Mariner-land, here’s an overview, with links for those interested, for the defensive statistics that I lend some credence to, and how I attempt to put them together to get an overall idea of a player’s contributions with the glove.