A metric that determines how effective a fielder is (outfield or infield) at avoiding errors. The resulting value is an indication of how many runs a certain fielder has gained or lost for his team compared to what an "average" fielder would do with the same number of fielding chances.
To calculate ErrR, we essentially look at the error rate of the individual player we are evaluating. Error rate is the ratio of errors to total fielding chances. We can then subtract the league average error rate from a player's individual error rate to yield that player's ErrR. All the possible scenarios are analyzed to determine their run value, so the final ErrR number is a net gain/loss of team runs.
Let's say, for example, we have a fielder who has had 200 fielding chances during the season, and has made 4 errors. If the league average at his position is 7 errors per 200 chances, he would have an excellent (positive) ErrR value.