EVALUATING BASEBALL PITCHERS AND
FORECASTING FUTURE PERFORMANCE
In chapters 2–4 we discussed three methods that can be used to evaluate the performance of a baseball hitter: Runs Created, Linear Weights, and Monte Carlo simulation. Let's turn our attention to evaluating the performance of baseball pitchers. As we will see, evaluating their performance is no easy matter.
Until recently, the most frequently used technique for evaluating the performance of pitchers was earned run average (ERA). Let's consider a pitcher, again named Joe Hardy. Consider all the runners Joe allows to reach base. Any of the base runners who score or would have scored if Joe's team made no fielding miscues (such as an error, passed ball, and so forth) causes Joe to be charged with an earned run. For example, if Joe gives up a triple with two outs in an inning and the next batter hits a single and a run is scored, Joe is charged with an earned run. Now suppose instead of a single the next batter hits a ball to the shortstop, who misplays the ball and is charged with an error. If the runner scores, this is an unearned run because without the error the runner would not have scored. A pitcher's ERA is the number of earned runs he gives up per nine innings. For example, if Joe gives up 20 earned runs in 45 innings, he has given upruns per nine innings and thus his ERA is 4. In general, a pitcher's ERA is
There are several problems with evaluating pitchers by their ERA.
1. Errors are subjective. Some official scorers are more reluctant than
others to call a batted ball an error. David Kalist and David Spurr have