# Archive for September, 2020

30th September 2020

Just in time for the 2020 postseason, Baseball-Reference has added championship win probability added (cWPA) and championship leverage index (cLI) to the site.

Just as single-game win probability added (WPA) measures how a player impacts their team's chances of winning a game, cWPA measures how a player impacts their team's chances of winning the World Series. Similarly, championship leverage index uses the same concept of single-game leverage index (LI), but expands the scope to measure the importance of a particular play, in how it impacts a team's chances of winning the world series.

These stats are highly dependent on context and are best used as "story stats" rather than determining which player was better. When telling the story of the history of baseball, we point to the greatest moments such as Bobby Thomson's Shot Heard 'Round the World, Bucky Dent's home run over the monster, David Freese's clutch performance in Game 6, or Madison Bumgarner's Game 7 performance. Moments like these are captured in cWPA and cLI, but it's not just history's greatest moments. Every event in our play-by-play database has a value.

How are cWPA and cLI calculated?
Let's look at Bobby Thomson's Shot Heard 'Round the World for example. This was the third and final game of the National League tiebreaker series. A win for the Giants would clinch the pennant with a 50% chance of winning the world series. However, a loss would end their season, meaning a 0% chance of winning the world series. The difference between a win and a loss in this game is 50%. To get the championship leverage index, we simply divide .5 by our baseline of .006 (The baseline is explained here). This means that the Giants' cLI for the game is 83.33 (.5/.006). The LI for Bobby Thomson's final at-bat was 4.74. To get the cLI for the at-bat, we simply multiply the game cLI by the at-bat's LI, which gives us 395.0 (83.33*4.74). This mean's that this at-bat is 395x more important to the Giants' chances of winning the world series than the average play on opening day.

When Thomson stepped to the plate, the Giants were down 3-1 with 1 out in the bottom of the 9th, giving them just a 29% probability of winning the game at the start of the at-bat. Since the home run ended the game, the probability of winning the game at the end of the at-bat was 100%. To get the cWPA for the play, we multiply the difference between game win probability at the start and end of the at-bat by the difference between the championship win probability of a win and a loss. This gives Thomson .355 cWPA ((1.0 - .29) * (.5-0)). This means that Thomson's home run increased the Giants' probability of winning the world series by 35.5 percentage points. On the flip side, the opposing pitcher Ralph Branca is given -.355 cWPA for the play.

Note: cWPA values are displayed in percentage format, so the example above displays as 35.5%.

There are currently a number of places to find cWPA and cLI on Baseball-Reference:
Batting and Pitching Game Logs: Yaz's amazing 1967 season
Batting and Pitching Win Probability Tables: Sandy Koufax's Pitching Win Probability
Postseason Series Pages: 1991 World Series
Box Scores: 1960 World Series Game 7
League Batting and Pitching Win Probabiliy Pages: 2020 MLB Batting cWPA
Team Batting and Pitching Win Probability Tables: 1975 Reds Batting
Team Schedules: 1978 Yankees

If you have any questions or feedback on this new feature, feel free to contact us through our feedback form.