Ultimate Zone Rating (UZR) is the most effective and comprehensive measurement of a player’s defensive ability. The statistic judges what the chances are that a ball in play will be able to be converted into an out depending on how hard the ball was hit, what zone it was hit to and whether it is a ground ball, fly ball or line drive.
That probability is then adjusted to take into account the park the ball was hit in, how many and where people were on base, the handedness of the batter, the pitcher’s groundball-to-flyball ration and how many outs there were at the time.
Fan Graphs updates the UZR over the course of the season.
