When a player’s offensive production is being discussed, people usually start with the usual stats. Batting average, OBP, SLG, OPS, etc, etc.
And then they get around to baserunning, and to the value of a stolen base.
So – to use Jemile Weeks of Oakland as an example – they will say “he had 22 SBs with only 11 CS – that adds additional value that is not reflected in his other offensive statistics”
But does it?
Consider the case of a lead-off single. One runner on base, nobody out. If you look at an expected runs matrix, the expected runs for that inning are about 0.9.
Suppose the runner successfully steals second. We now have a runner at second with nobody out, giving an expected runs of 1.14. Or, to put it another way, the successful steal of second added 0.24 of an expected run.
Suppose, on the other hand, that the runner tries to steal and is thrown out. With one out and nobody on base, the expected runs decline from 0.9 to 0.28, for a decrease of 0.62 expected runs.
So for every successful steal, the team “gains” 0.24 expected runs, and for every unsuccessful steal the team loses 0.62. It follows that, to break even, a baserunner would have to succeed roughly 72% of the time.
Or to put it another way – when Jemile (in my example) stole 22 bases, he generated a probabilistic 5.3 runs. The 11 times he was caught stealing cost his team a probabilistic 6.8 runs. So, on this basis, his 22-11 record hurt the term rather than helping it.
Of course, the above calculation are simplistic. They do not take into account how a baserunner can disrupt the pitcher’s concentration, or how a runner starting to steal can stay out of a double play. But neither do they take into consideration the double plays caused by a fielder catching a line drive and throwing the runner (already at second) out at first. Nor do they show the effect of a stealing baserunner allowing the first baseman to play off the bag, making it harder for the batter to get a ground ball through to right field.
And in case you think that 72% is an unreasonable target: how many of the top 10 basestealers in 2011 do you think achieved a 72% or better success rate?
Answer: all of them.