A Statcast metric designed to use data to evaluate the performance of baserunners and outfielders in taking or preventing extra bases.
Based on inputs including runner speed, outfielder throwing arm, runner position on the base paths, and outfielder distance from both ball and bases, an estimated success probability can be created for each opportunity. With that information available for each play, the player’s actual success rate can be compared to the estimated rate and the cumulative metric can be created. Importantly, it does account both for bases taken (prevented, for outfielders) and chances taken or not. In that way, credit can be given to outfielders who do not get a chance to make an assist because their reputation for quality throwing precedes them.
This does account for extra bases taken by batters or runners on batted balls; it does not include stolen bases, as it’s about taking extra bases against fielders.
A good example can be seen here, when Ronald Acuña Jr. threw out Alec Burleson attempting to stretch a single into a double. Based on factors including the location of the ball, the distance from second base, and knowledge of the speed and location of the runner, this was a chance where Acuña was given just a 5% estimated likelihood of throwing the runner out. That he successfully did so gave him +0.95 runs to his seasonal total.
The top of the 2022 seasonal leaderboard offers a good example of how different skills can lead into the seasonal total.
Michael A. Taylor led at +6 runs, ahead of several tied at +5. Taylor’s +5 came via: +4 runs on throwing runners out, +6 runs on holds (having runners not attempt to advance), and -5 on advances (allowing runners to advance). In that way, he receives credit for more than just assists.