Base running linear weights or base running runs, or Ultimate Base Running (UBR), is similar to the outfield arm portion of UZR. Whatever credit (positive or negative) is given to an outfielder based on a runner hold, advance, or kill on a batted ball is also given in reverse to the runner (or runners). There are some plays that a runner is given credit (again plus or minus) for that do not involve an outfielder, such as being safe or out going from first to second on a ground ball to the infield, or advancing, remaining, or being thrown out going from second to third on a ground ball to SS or 3B.
Runs are awarded to base runners in the same way they are rewarded to outfielders on “arm” plays. The average run value in terms of the base/out state is subtracted from the actual run value (also in terms of the resultant base/out state) on a particular play where a base runner is involved. The result of the subtraction is the run value awarded to the base runner on that play.
If you didn’t understand that, a simple example should explain it clearly:
Let’s say that there is a runner on second and one out. A ground ball is hit to the SS. Let’s say that on the average, in that same situation, the runner advances safely to third and the batter is thrown out 20% of the time, he stays put 70% of the time, he gets thrown out at 3rd 5% and beats a throw to third 5% of the time (batter safe on a FC). And let’s say that average base/out run expectancy (RE) of all those results, weighted by their frequency of occurrence, is .25 runs (all the numbers are made up). If the runner advances and the batter is thrown out, and the resultant RE is .5 runs, then the runner gets credit for .25 runs (.5 minus .25). If he stays put, and the average RE of a runner on second and 2 outs is .23 runs, then gets “credit” (he gets docked) for -.02 runs (.23 minus .25). So basically a runner gets credit for the resultant run value of what he does minus the average weighted resultant run value of all base runners in that situation.
Here are most of the situations where a base runner gets some kind of positive or negative credit. Obviously more than one base runner can get credit on any particular play.
1) On a hit, advancing an extra base, not advancing an extra base, or getting thrown out trying to advance an extra base, as long as no other base runner is blocking an advance.
2) A batter getting thrown out trying to advance an extra base on a hit (if he successfully does, we don’t know it, as he is simply awarded a double, for example, on a usual single where he advances an extra base).
3) On a hit, the batter advancing, not advancing, or getting thrown out when a runner is safe or out advancing an extra base.
4) Trailing runners advancing, not advancing or getting thrown out when a leading runner is safe or out trying to advance an extra base on a hit or an out. This is basically lumped together with #1 above.
5) Runners trying to advance on fly ball outs – i.e. tagging up.
6) As mentioned above, on ground balls to the infield, runners on first staying out of the force or DP at second base, whether the batter is out or is safe on a FC.
7) Also as mentioned above, a runner on second advancing or not (or getting thrown out) on a ground ball hit to SS or 3B.
Runners on third base advancing, not advancing, or getting thrown out at home on a ground ball are not considered (on air balls they are). Runner advances or outs on WP or PB are not considered either.
All of these situations are considered an “opportunity” for the base runner or the batter (except when a batter gets a hit, he is not awarded an opportunity unless a leading runner tries to advance an extra base and the batter has an opportunity to advance on the throw).
As with UZR, a player’s “games” are not his actual games played. They are his opportunities divided by the league-average opportunities per game. No adjustments are made for how often a player typically gets on base (or how often subsequent batters put the ball in play or get hits), such that a player with a high OBP will likely have more “games” than actual games played and a player with a low OBP will likely have fewer “games” than actual games played.
On batted balls to the outfield, only whether it was hit to LF, CF, or RF is considered, not the depth or actual vector or zone within the three outfield positions.
Future versions of UBR will likely adjust “games” for a player’s OBP and will also likely include more outfield location zones. These upgrades are not likely to significantly change the numbers.