GUIDES · BETSLIP DOCTOR
Why can identical hits receive different win probability added values?
WPA depends on the game state around the hit. The same single can change a team’s win expectancy by different amounts in different innings, score situations, out counts and base states.
For Bettors and cappers learning to read baseball records · Updated
The game state changes the before-and-after value
Win expectancy is a model-based estimate derived by comparing a specific game situation with similar historical situations. Score, inning, outs, runners on base and run environment matter. WPA is the team’s win expectancy after the play minus its expectancy before the play.
That is why the same result at the plate can receive different WPA in two situations: it changes the model’s before-and-after state by a different amount.
Use WPA as retrospective context
WPA helps describe how a completed event changed the modeled outlook during that game. It is not a prediction that the team will win, a promise about a later result or a new betting signal. The value depends on the win-expectancy model and its historical reference.
One fictional single in two states
Hypothetical State A: the home team is tied in the third inning, with one out and a runner on second. A single scores that runner. Hypothetical State B: the home team trails by one in the bottom of the ninth, with one out and a runner on second; the same single and runner movement ties the game. A sourced WPA model would compare its before-and-after win expectancy in each state. The second change may be larger because the game is later and closer, but no numerical WPA is claimed here.
Related questions
Does positive WPA guarantee that the team wins?
No. It records a modeled change in win expectancy at that event; later events can change the outlook again, and the team can still lose.