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DM 091-Yankees and Red Sox : A Time Series Analysis of Win Percentage Correlations between Professional Baseball Teams

Objective. The purpose of this study is to demonstrate how time series can be used to analyze the correlations of win percentages between the Red Sox and Yankees rival professional baseball teams. Method. The data to be analyzed in this paper were collected from Baseball-Reference.com or were calculated from data from said resource. The data of primary interest include wins and win percentages for each team. Wins were projected as if each season consisted of the current standard of 162 games. The relevance of additional inputs in creating the time series model was investigated, including a study of head-to-head match-ups between the two teams and the sale of Babe Ruth from the Red Sox to the Yankees. Results. The ratio of the Yankees winning percentage to the Red Sox winning percentage begins to more closely approximate 1.0 starting in 1968, paralleling a general historical trend of increased competitiveness in Major League Baseball. The Yankees and Red Sox have a sharp increase in wins in 1994 and 1995 respectively. These factors have continued to ensure that this rivalry is arguably the greatest in all of sports. Both head-to-head match-ups between the teams and the sale of Ruth seem to help us obtain a more accurate model, but these results are tempered by a conflicting outcome when a hold-out sample is used. Conclusion. The methods shown here can be applied to win percentages of professional baseball teams to analyze competitiveness, likelihood of rivalry and historical winning trends. This work could be extended to measure the effect of an acquisition of a player, a geographic move, a change in ballparks, etc.