ERA+: Looking Behind The Stat

ERA+ is a great analytical tool. It permits comparisons of ERAs across different eras and different run environments by adjusting for general league scoring levels and park factors. Its advantages over simple ERA are obvious. It is the single pitching statistic most often regarded as the definitive tool for analyzing pitching careers. Some stat geeks have become so enamored of ERA+ and its derivatives that they deny certain baseball truisms that might call into question the validity of judging pitchers primarily on the basis of ERA+. They tend to deny the concept of clutch pitching, despite the fact that certain pitchers evince a tendency to pitch measurably better or worse in high leverage situations (see this post for a discussion of Leveraged ERA+, or LevERA+, which weights runs allowed (and runs prevented) based on the impact on win expectancy). They also tend to discount the theory that most pitchers "pitch to the score" by changing their pitching approach depending on the game situation.

A host of statistics confirm that most pitchers do indeed pitch to the score. Pitchers as a group adhere to the theory that when granted a big lead it is better to put the ball over the plate and make the opposition hit their way back into the game rather than risking a rally fueled by putting runners on base via bases on balls. Virtually all successful pitchers walk fewer batters when working with a significant lead. Virtually all pitchers, successful or not, surrender more runs when working with tremendous run support from their teammates. Baseball-Reference.com recently added pitching splits based on team run support, showing a pitcher's performance in games in which they received between 0 and 2 runs of support, 3 to 5 runs of support and 6 or more runs of support. The vast majority of pitchers will surrender more runs on average when working with 6 or more runs than they do when working with 5 or fewer runs. The run-support splits further confirm that variations in ERA in high run-support scenarios have little or no impact on a pitcher's winning percentage in these scenarios, with good pitchers winning between 90% and 95% of these decisions regardless of how much their ERAs increase with great run support.