https://fivethirtyeight.com/features/is ... questions/
Getting better, but still not there yet.
Can we finally measure defense?
Since the inception of sabermetrics, defense has always stumped the statheads. Without the detailed data — like pitch location and exit velocity — that’s available to measure pitching and hitting, defensive metrics have been unreliable and inaccurate. Adam Eaton was one of the best fielders in baseball in 2016 … and, according to those same metrics, a below-average defender in 2015.
But the future of defensive stats looks brighter: MLB’s new Statcast system can measure everything about a defensive play, from the running speed of the fielder to the exact landing point of the ball. Armed with that new data, MLB’s statisticians have crafted impressive new metrics to quantify the difficulty of every outfield catch over the last two seasons, a huge upgrade on the information we had available before. (Kevin Kiermaier, your Gold Gloves appear to be well deserved.)
Still, the stats aren’t perfect. They don’t account for the direction the fielder has to run in, which means that they treat running forward the same as backpedaling. They don’t incorporate any information about an outfielders’ throws, so a strong and accurate arm counts for nothing. And they are only available for the outfield. Statcast still has major issues tracking grounders (losing as much as 20 percent of all balls in the dirt), so for now, the much more complex mystery of infield defense remains unsolved.
Perhaps the biggest problem with these defensive statistics is that they are not being released in full to the public. While MLB is providing snippets of the data in leaderboards and tweets, the complete data set is being kept under wraps. Front office insiders I’ve spoken to have pointed to issues with the data’s quality and the influence of teams eager to keep their analytics edge as two barriers to the data set’s full release. At a crucial point in Statcast’s development, it’s reasonable to wonder whether the data stream will eventually become fully public (like PITCHf/x) or whether it will remain a tool primarily for the front office (like the NBA’s SportVU camera system). –Rob