Last night, Jacoby Ellsbury pulled off the rare play of a straight steal of home. The feat electrified the Fenway crowd, but was it a good play? It was the bottom of the 5th with the Red Sox leading 2-1. The Yankees' southpaw Andy Pettitte had just intentionally walked Kevin Youkillis to get to JD Drew to load the bases with two outs. Pettite threw a fastball for a swinging strike one, then a breaking ball outside for a ball. Then Ellsbury took off....

Let's look at the factors which affect the chances that a player is able to steal home or not and whether Ellsbury had them in his favor.

1) Speed of the runner. Obviously, this is vital and Ellsbury has great speed.
2) Pitcher's stance. It's far easier to steal home if the pitcher is working from a windup - Pettitte was.
3) Pitcher's handedness. A lefty turns his back to third during his windup, meaning he can't see the runner take off. Pettitte's a lefty.
4) Batter's handedness. It's easier to steal home with a righty at the plate, since he blocks the catcher's view to third base. With a lefty, he can see the runner coming. Drew was a lefty, which was a drawback for Ellsbury.
5) Pitch selection. Obviously a curve or a change-up are the best pitches to run on since they take longer to get to the plate. Previously, Pettitte got a fastball for a swinging strike one and threw a breaking ball for a ball. Ellsbury guessed right on the third pitch as Pettite threw a big slow curve ball.
6) Attention. In order to steal home, the defense has to be oblivious to it. The third baseman was playing well off the bag, and Pettitte paid no attention to Ellsbury. He was able to get an enormous jump down the third base line.

Overall, Ellsbury had 5 of 6 factors in his favor, meaning he had a decent chance to pull off what's become an increasingly rare feat. However, did the game situation call for a steal of home? Let's look at the factors relating to this.

1) Score/Inning. The best time to run is late in the game when the game is tied or you are down by one. The Red Sox were up by one in the 5th, which wasn't ideal.
2) Outs. The play must be done with two outs, since with less than two outs there are plenty of easier ways to get a man home from third. There were indeed two outs in the inning.
3) Other runners. Ideally, nobody else is on base - that way you don't take yourself out of a potential big inning if you get thrown out. The Red Sox had the bases loaded, which means Ellsbury was really gambling by running.
4) The batter. A weak hitter at the plate is ideal since it makes it harder for the runner to score by means other than a steal of home. JD Drew is a good (but not outstanding) hitter, so Ellsbury was also gambling by potentially taking the bat out of his hands.
5) The count. A pitcher's count is best since it limits the chances that the runner can score by other means. The runner can't go on two strikes since the batter must swing, so an 0-1 count is ideal. Ellsbury ran on 1-1, which isn't great, but better than a 2-1 or 3-1 count.

Ellsbury only had 1 out of 5 of these factors really in his favor, meaning while he might be capable of stealing home, it would be a risky play. From a WPA perspective (not taking into account batter or count), the Red Sox had a 72.1% chance of winning before the steal. Afterwards it increased to 79.8%. Had he been thown out, the chances would have dropped to 65.8%. The break-even point for the steal was 45%, meaning that if Ellsbury were safe 45% of the time, it would be a good play.

Stealing home is so difficult, that ideally all 11 factors that I outlined would have to be in a runner's favor before attempting a straight steal of home. Ellsbury had only about half working for him in this case, meaning that while exciting, it might not have been the smartest baseball play ever. But Ellsbury beat the throw (and beat it fairly easily), so it's hard to argue with results - perhaps he knew something we didn't. In any case, cheers to him making the most exciting play in baseball thus far in 2009.