Defense statistics have long been the bane of saberists everywhere. While offense and pitching stats are easy to quantify because they result from distinct, calculable events, how do you easily measure a player’s range, positioning, arm strength, and error rate? These sort of measurements are complicated and involve lots of work to ensure accuracy.
In recent years, though, we’ve seen two fielding metrics rise above the rest of the field and establish themselves as reliable: the Dewan Plus/Minus system (AKA, Defensive Runs Saved) and Ultimate Zone Rating (UZR). Both systems don’t always agree, but when compared with each other and taken in large sample sizes (~3 years), you can get a good idea of a player’s fielding abilities. Also, despite the common perception that fielding statistics are unreliable, UZR correlates year-to-year at almost as high a rate as wOBA, which is regarded as one of the most reliable offense statistics available. In other words, fielding stats have come a long way in a very short time.
The one area that fielding statistics still haven’t reached is catcher defense. Some efforts in this regard have been made recently, but it’s very difficult to quantify things like pitch framing and game calling.