A metric that evaluates the impact an outfielder has on the game with his throwing ability. The rating of outfielders is based on their ability to keep runners from advancing on the bases, as well as their ability to throw out runners who are attempting to advance.
Outfield Arm Runs is another statistic that is based on the number of runs a player gains or loses for his team. The average performance of an outfielder in various situations is subtracted from the performance of a particular outfielder in those same situations, thus giving us a positive or negative ARM rating. These scenarios are analyzed to determine their run value, so the final ARM number is a net gain/loss of team runs due to an individual outfielder's ability to throw.
One example of this is as follows: let's say the offensive team has a runner at first base. The batter hits a ground ball single to right field. In many instances, the runner would be able to advance to third on a play like this, but in our example, we'll say the right fielder makes a hard and accurate throw to third base, forcing the baserunner to stop at second base. The player who made the throw would be credited with a positive ARM value on this play, since on average, the runner should have been able to reach third in this scenario.