A metric that examines the positive and negative effects of a baserunner's actions. The resulting value is an indication of how many runs a certain baserunner has gained or lost for his team compared to what an "average" baserunner would do in the same situations.
In calculating UBR, each possible combination of runners on base and number of outs is analyzed and given a run value. From this, the average run value for all the particular scenarios can be determined. UBR for a particular player can then be calculated by finding the difference between the player's actual outcome and the average outcome in those same situations.
As an example, let's say there is a runner at first, with one out. The batter hits a slow roller to third base, and the batter beats the throw to first base and is credited with an infield single. The runner, recognizing that the play at first is slow in developing and knowing he has excellent speed, advances all the way to third base on the play. In most instances, a runner would only advance to second base on a play like this, so the baserunner in our example would earn a high (positive) UBR on this play.