Ever wonder how often a .550 team beats a .450 team? I did, and after a little searching I stumbled onto the Log5 equation. Introduced by Bill James in his 1981 Bill James Baseball Abstract, it states: A - A * B WPct = ----------------- A + B - 2 * A * B Where: A = Team A's Winning % B = Team B's Winning % WPct tells us what % of games team A will beat team B, on average. What if we want to calculate the odds of winning 1 game in a 2 game series, however? The first step is to calculate the odds of winning 1 game "one way". The odds of winning the first and losing the second: PROB = WPct * (1 - Wpct) WPct is the probabilty of winning the first game and 1- WPct is the probabilty of losing the second game. The probability of BOTH occuring is simply the product. But that's only one way to win 1 game of a 2 game series, however. The team could lose the first and win the second and still win 1 game in the series. Common sense tells that that the probability of winning a specific number of games in a series is then: PROB(one way) * number of ways So, the probabilty of winning 1 game in a two game series is: WPct * (1-WPct) * 2 If we want to calculate the probability of winning any number of games in any length of a series we need to create a more flexible equation. We'll define the two variables we need as: G = total games in the series W = number of wins in a series The probabilty of winning "one way" is then simply: [WPct ^ W] * [(1-WPt) ^ (G-W)] All that's left to figure out is the "number of ways" part. We could figure it by hand if we wanted to. For a 3 game series, the number of ways to win 1 game is: WLL LWL LLW Three ways to win 1 game in a 3 game series. It just so happens that the Combination function can calculate just how many ways you can win any number of games in any length series: "number of ways" = C(G, W) Putting the final equation together: PROB(G,W) = [WPct ^ W] * [(1-WPt) ^ (G-W)] * C(G, W) A few more points to finish this post. Since the home team wins 4% more games, simply add .04 to their WPct. Also, I use the pythagorean winning percentages for the Log5 formula since I feel it predicts future success better. If anyone is interested in the actual computer code I will be happy to type it in. Let me know if there's any questions or suggestions! Links: Estimating one-game winning percentages Combination Formula Pythagorean Theorum