Calculating win probabilities

April 20, 2022


Statistics  /  , ,

Zack Capozzi, for USA Lacrosse Magazine, explains how he calculates win probabilities pre-game and during games. On interpretation, which could easily apply to other sports and all forecasts:

But interpretation here matters quite a bit. And this is frustrating for some people, but that 61 percent should be interpreted as: “if these teams played 100 times, we would expect Marquette to win 61 of those games.” It definitely does not mean that the model is 61 percent confident that Marquette will win.

This is a bit odd, but this also means that if the Win Probability model gives Team A a 90% chance to beat Team B, there is nothing wrong with the model if Team B ends up winning the game. The issue would arise if, out of 100 90-percent win probability games, the favorite wasn’t winning around 90 of those games. When the model says 90 percent, you want it to mean 90 percent.

I wonder how many people incorrectly interpret the probability as “61 percent confident”. I bet a lot.

I do know that ever since the Golden State Warriors lost to the Cleveland Cavaliers in the 2016 NBA Finals — while holding a 90-something percent win projection by FiveThirtyEight — I stopped paying attention to win probability. But learning more about the calculation made it more interesting.