Baseball-Reference’s Sean Forman introduces and explains the statistic Win Probability Added for the New York Times:
For postseason play, this statistic has the advantage of naturally taking into account clutch and unclutch performance because the context of the play is ingrained in the statistic. We can even create charts detailing how the win probability changes over the course of the game.
Interesting factoid I learned from reading this: By Win Probability Added (or, in this case, subtracted), Mets shortstop Rey Ordonez had the worst postseason of all time in 1999. Ordonez hit .132 with no walks and one extra base hit in ten games. Most of the damage was done in the Robin Ventura “grand slam single” game, when Ordonez went 0-6 and hit into a bases loaded double play in the sixth. I remember Rey Rey being an awful hitter, but sometimes it’s fun to look at his numbers and really remember how awful he was.