Win Probability and Leverage Index

A new stat that I have recently found is Win Probability Added (WPA) from Baseball-Reference.com. What this stat essentially measures is how much a certain team’s probability of winning increased or decreased during a particular play. For instance, a solo home run in a close game in the 9th inning will have a much higher WPA than a solo home run in a 6th inning blowout. The reason this stat is so useful is because it takes game context into account when measuring the value of a player. Other stats that are involved with this are Leverage, Leverage Splits, and Winner’s Win Expectancy, all of which can be further examined by following the above link. Another great place to look at Win Expectancy and Leverage Index is another one of my new favorite sites, fangraphs.com.

One fun thing to do with this type of stat is to look at a box score from Baseball-Reference.com, where they have the probability of the eventual winner going on to win the game before and after each play. You can see the top five most important plays of the game and follow who had the largest effect on a team winning or losing the game. One interesting game to look at is the August 5, 2001 match-up between the Mariners and Indians, where the Mariners jumped out to a 14-2 lead after 5 innings, and the Indians came back to score 12 runs in the last three innings on their way to a 15-14, 11-inning win. Below is a graph of the running probability that Cleveland would go onto win the game.



As you can see, the probability of the Indians actually winning this game was below 1% from the middle of the 2nd inning to the middle of the 7th.The large jump in the bottom of the ninth is due to Omar Vizquel’s game-tying bases loaded triple with two outs. Before that hit, the Indians had a 10% chance of winning, and after the play, the Indians had a 63% chance of winning. Thus, the WPA for that play is 63% - 10% = 53%. This value is credited to Omar Vizquel and debited to the pitcher, Kazuhiro Sasaki.

For those who are interested, one of the creators of this statistic looks at the infamous top of the 8th of game 6 of the 2003 NLCS. I blame Alex Gonzalez.

Interesting baseball fact of the day: David DeJesus had the largest single-play WPA of the 2008 season, when he hit a two-out, two-run walkoff homer against the Mariners for a 5-4 Royals win. The hit took the Royals from a 9.6% probability of winning to 100% probability of winning, for a 90.4% increase. Manny Ramirez led the league in net WPA, with a season-long WPA of 7.57. The player with highest average Leverage Index, meaning the player who was always in the game in intense situations, was Francisco Rodriguez, with an average LI of 2.54 (below 0.7 is low, 0.7-1.5 is medium with the average at 1.0, and above 1.5 is high leverage).