Description:

In the words of David Appelman, Win Expectancy (WE) is, “the percent chance a particular team will win based on the score, inning, outs, runners on base, and the run environment.” These percentages are calculated using historical data, meaning if your team is down and has a 24% win expectancy, only 24% of teams in similar situations in the past have ever come back to win.

Context:

As a game goes on, the win expectancies for both teams are constantly changing on a play-by-play basis.  This is what a win expectancy chart for a game looks like:

This is the chart of the last game of the 2009 World Series.  Notice how as the game goes on, the green line moves closer and closer to the Yankees’ side of the chart, indicating that the Yankees are becoming more and more likely to win.  Also notice that certain key events change the win expectancy more than others (see: Win Probability Added).

Things to Remember:

- To get a feel for how win expectancies work, check out Tom  Tango’s Win Expectancy Charts.

Links for Further Reading:

Get to Know: Win Expectancy – FanGraphs

The Game State, Run Expectancy, and Win Expectancy – Lookout Landing

Misunderstanding Win Expectancy – The Book Blog