Application of the Pythagorean Expected Wins Percentage and Cross-Validation Methods in Estimating Team Quality
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The Pythagorean Expected Wins Percentage Model was developed by Bill James to estimate a baseball team expected wins percentage over the course of a season. As such, the model can be used to assess how lucky or unfortunate a team was over the course of a season. From a sports analytics perspective, such information is valuable in that it is important to understand how reproducible a given result may be in the next time period. In contest theoretic (game theoretic) parlance, the original model represents a (restricted) Tullock contest success function (CSF). We transform, estimate, and compare the original model and two alternative models from contest theory, the serial and difference form CSFs, using MLB team win data (2003 to 2015) and perform a cross-validation exercise to test the accuracy of the alternative models. The serial CSF estimator dramatically improves wins estimation (reduces root mean squared error) compared to the original model, an optimized version of the model, or an optimized difference form model. We conclude that the serial CSF model of wins estimation substantially improves estimates of team quality, on average. The work provides a real world test of alternative contest forms.
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