Outs Above Average (OAA) is a range-based metric of skill that shows how many outs an outfielder has saved over his peers, accounting for not only the number of plays an outfielder makes (or doesn't), but also the difficulty of them. (Leaderboards are available for both individuals and teams.)
OAA starts with Catch Probability, which takes the distance an outfielder must go, the time he has to get there, and the direction he travels to put a percentage of catch likelihood on each individual batted ball. Outs Above Average is the season-long cumulative expression of each individual Catch Probability play. For example, if an outfielder has a ball hit to him with a 75 percent Catch Probability -- that is, one an average outfielder would make three-quarters of the time -- and he catches it, he'll receive a +.25 credit. If he misses it, he'll receive -.75, reflecting the likelihood of that ball being caught by other outfielders.
For example, the play shown below was considered an extremely difficult one with a 10 percent Catch Probability, so Harrison Bader received +.90 for making a catch that few others do.
Adding up credit for every play made or not made contributes to a seasonal Outs Above Average number. In 2016, Billy Hamilton led the Majors with +24 OAA. Byron Buxton did the same with +24 OAA in 2017.
In addition, the leaderboards also show columns for "Expected Catch Percentage," "Actual Catch Percentage," and "Catch Percentage Added," providing the ability to see how a player has performed on a rate basis.
• Expected Catch Percentage shows how many plays an average outfielder would be expected to come up with based on the difficulty of the batted balls hit to the outfielder in question. This is a good proxy to see if the fielder is getting easy or difficult opportunities. Of the outfielders with at least 150 chances in 2017, Mike Trout's Expected Catch Percentage of 89 was the highest, meaning an average outfielder catches 89 percent of the balls Trout saw. This shows he got the least challenging opportunities sent his way, while Norichika Aoki, at 77 percent, got the toughest.
• Actual Catch Percentage shows the production of the actual fielder on the balls hit his way. Given the difficulty of batted balls, Trout would be expected to make the catch 89 percent of the time, but in actuality, he made the catch 88 percent of the time.
• Catch Percentage Added is the most important, because it's the difference between the Expected and Actual numbers. For example, Heyward and Ben Gamel were each given the same difficulty of opportunities, being expected to catch 86 percent. But with an Actual Catch Percentage of 90, Heyward added four points of value. Meanwhile, Gamel's Actual Catch Percentage was 83, so he subtracted three points of value. Heyward was seven points more valuable on a rate basis than Gamel, and +18 (comparing his +10 to Gamel's -8) Outs Above Average on a cumulative basis.