Log5 is a method of estimating the probability that team A will win a game against team B, based on the odds ratio between the estimated winning probability of Team A and Team B against a larger set of teams.
Let and be the average winning probabilities of team A and B and let be the probability of team A winning over team B, then we have the following odds ratio equation
One can then solve
The name Log5 is due to Bill James but the method of using odds ratios in this way dates back much farther. This is in effect a logistic rating model and is therefore equivalent to the Bradley–Terry model used for paired comparisons, the Elo rating system used in chess and the Rasch model used in the analysis of categorical data.
A few notable properties exist:
- If , Log5 will always give A a 100% chance of victory.
- If , Log5 will always give A a 0% chance of victory.
- If , Log5 will always return a 50% chance of victory for either team.
- If , Log5 will give A a probability of victory.
- "Chancesis: The Origins of Log5". Archived from the original on April 12, 2012. Retrieved 2013-03-07.
- "Baseball, Chess, Psychology and Pychometrics: Everyone Uses the Same Damn Rating System". Retrieved 2013-12-29.