Log5

The Log5 method is Bill James' name for a method of estimating the winning percentage when two teams meet, or the expected rates resulting from a batter/pitcher confrontation. While James developed the team method independently and the batter/pitcher method in conjunction with Dallas Adams, the team method is in fact equivalent to the Odds Ratio method and can be derived from Bayes formula.

Log5 for Teams

The expected winning percentage of team A versus team B is given by:

W%(A v. B) = W%(A)*(1 - W%(B))/(W%(A)*(1 - W%(B)) + (1 - W%(A))*W%(B))

Incorporating Home Advantage

To add a home-field/court/ice advantage term to the log5 formula, plug the expected probability of a home team winning an even matchup (for instance, .60 for the NBA, .55 for the NHL, .54 in MLB) into the following formula:

((Team A win%) * (1 - Team B Win%) * HA)/((Team A win%) * (1 - Team B Win%) * HA +(1 - Team A win%) * (Team B Win%) * (1 - HA))

Where Team A is the home team, Team B is the road team, and HA is the home advantage.