FanGraphs Game Odds!

For every game this year we’ll be projecting the odds of each team winning on our live scoreboard. We’ve also added an extra experimental line on live Win Probability graphs that adjusts for the newly calculated game odds.

To calculate these game odds we’re doing the following:

1. First we take a combination of the ZiPS and Steamer projections for each team’s position players (or starting lineup once that information becomes available), and each team’s starting pitchers and bullpen.

2. We take those projections and run them through Base Runs and then the PythagenPat to get a winning percentage.

3. Then we take each team’s expected winning percentage and run it through Log5 to get the actual winning percentage of the home team.

4. Finally, we add 4% to the home team, to account for home field advantage.

To calculate the new line on the Win Probability graph, things get a little more complicated.

1. First we add 1 run to the team we have projected to win to find how much weight to give the regular win probability line and the new win probability line where the projected winning team is up by 1 run.

For instance, at the beginning of a game, the non-projected win probability is always 50%. If we project the home team to have a 57% chance of winning, and the win expectancy of a team at the start of the game being up by 1 run (with no outs and no runners on base) is 60%, we would weight the 50% win expectancy by 3 and the up by 1 run win expectancy by 7. That will be our baseline weighting for the rest of the game.

2. Then for each out, we push the weighting towards the 50% line for each out that is made. In the end, the lines converge at the actual winning percentage at the end of the game (100% for the winning team). Basically, as the game goes on, we continue to discount our projected odds.



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David Appelman is the creator of FanGraphs.


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