ARP

From Wiki Gonzalez

Adjusted Runs Prevented. A measure that tries to determine how many runs a reliever prevented adjusted for things like the runners inherited and runners bequethed if the pitcher enters or leaves a game mid inning as ERA doesn't do such a good job of measuring a reliever. To calculate:

(ExpRunsStartBaseOutPark - ExpRunsFinishBaseOutPark + InningsFinished*ExpectedRunsPerInningPark - Runs)/ParkFactor

where

ExpRunsStartBaseOutPark - Is the expected number of runs to score given the base runners and outs, in the given park, that the pitcher faced when he entered the game
ExpRunsFinishBaseOutPark - Is the expected number of runs to score given the base runners and outs, in the given park, that the pitcher left when he exited the game
ExpectedRunsPerInningPark - Is the expected number of runs to score in an IP given this park