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 a team is losing and has a 24% win expectancy, only 24% of teams in similar situations in the past have ever come back to win.
To get an in-depth feel for how win expectancy works, check out Tom Tango’s Win Expectancy Charts. That page breaks down the win expectancy for every sort of situation in tied or 1-run games.
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 win expectancy chart for Game Six of the 2011 World Series. Notice how as the game goes on, the green line moves closer and closer to the Rangers’ side of the chart, indicating that the Rangers are becoming more and more likely to win. That peaks in the ninth inning, when the Rangers came one strike away from winning the game before the Cardinals rallied to tie the game and then win it.
Those final innings were a whirlwind, and the bouncing win expectancy line mimics the path my stomach was taking over those final few innings. Win expectancy is a great story-telling stat.
Also, notice that certain key events change the win expectancy more than others (see: Win Probability Added).
Links for Further Reading: