Let’s Go Back to October

Every year when the postseason rolls around, we enjoyers of baseball try our best to make sure we’re properly appreciating the history unfolding on the field before us. We want to acknowledge when we’ve just watched a game so magical that it will be spoken of in tones of awe and disbelief for years to come. Downstream of that, we like to evaluate whether a game, a series, or even an entire postseason was a good one, mentally sorting them into tiers with other postseasons we’ve watched. Some measures of “good” are subjective, coming down to our personal preferences for certain strategies, styles of play, narratives, teams, or players. Other measures are more universally agreed upon and objectively quantifiable. In particular, most neutral observers value a close, exciting game, one that features both tension and action to keep observers engaged.
Win Probability Added (WPA) provides a reasonable proxy for measuring both tension and excitement. At the plate appearance level, it uses the score, inning, and base-out state (i.e. runner on second, two outs) to calculate a team’s win expectancy based on historical outcomes. The difference in a team’s win expectancy after a plate appearance relative to what it was before it represents the WPA during the plate appearance in question. WPA will be negative for the team whose odds of winning decreased while being positive for their opponent, but in this context, we’re going to focus on the magnitude of the change in win expectancy. Without a rooting interest, it’s less about which team wins and more about seeing big plays that impact the outcome of the game. Games with a large quantity of WPA have a lot of high-impact plays and lead changes that allow teams to pass win probability back and forth between one another.
Using WPA, we can evaluate the quality of the action in a given game by both looking at the average WPA per plate appearance and by adding up the game’s total WPA. Both methods provide useful insight. Average WPA per plate appearance controls for the variable number of plate appearances in a game, since games with more plate appearances have more opportunities to accumulate WPA. Sometimes that accumulation constitutes empty calories; other times it’s more substantial. Ultimately, we want the games that top the charts from both perspectives. Read the rest of this entry »