What are Net Runs?
Similar to Sean Smith's RE24, Net Runs scores each play in a game solely by the run expectancy added or subtracted by each player, and gives us a total number of runs contributed for each player in the game. It delves deeper than RE24 by awarding defenders for plays made in the field, awarding baserunners for their baserunning, and factoring in luck, managerial decisions that take matters out of the players' hands and the dimensions of the park. Pitchers are scored in full only for outcomes in their direct control (walks, strikeouts, wild pitches, etc), while balls in play they allow are mostly scored according to their MLB average run value, adjusted for park factors.
Run expectancy numbers are derived from a composite five year league average (2005-2009) weighed more heavily towards recent seasons, and adjusted by park using ESPN's park factors and Tom Tango's Markov function. 2010 data will get factored in accordingly (with 2005 data being phased out) as that sample builds and normalizes.
What is EXERA?
EXERA is short for Expected ERA, taken from a pitcher's Net Runs earned and added with the league average runs scored per inning in the respective park(s) to devise an ERA the pitcher would expect to run in that park with a league average defense based on his performance. Each park has it's own baseline EXERA, against which the pitcher's performance is compared.
If a pitcher's actual ERA is better than his EXERA, his defense helped him out and/or he had some luck. But if it's worse, he had some bad luck or poor defense.
What is EMERA?
EMERA is short for Expected MLB ERA and is used to estimate how a given minor league pitcher would fare if he were immediately called up to the Majors, based on his walk, strikeout and batted ball rates during a given season at his current level.
To estimate this performance, the major league averages for walks, strikeouts and the three major batted ball types are adjusted to the player's given level through MLE adjustments. Then the player's rate for each batted ball type is adjusted by these revised weights to create a hypothetical opponent's batting line over the pitcher's number of ABs against. This line is run through a Markov equation to devise a runs per game total, which is then adjusted to give us EMERA. EMERA can also be used to compare pitchers between levels and organizations, since their raw numbers are neutralized.