ERA, or earned run average, is the number of earned runs a baseball pitcher allows per nine innings. Earned runs are runs that scored without the aid of an error or a passed ball. ERA is a commonly accepted statistic for evaluating pitchers.
A lower ERA is better than a higher ERA. The lowest possible ERA is 0, which means that there were 0 earned runs.
The formula for finding ERA is:
For example, a pitcher who allows 5 earned runs in 7 innings pitched would have an ERA of 6.43 ().
If a pitcher exits a game with runners on base, then any earned runs scored by those runners will count against him.
Because the goal of pitching is to prevent runs from scoring, and ERA gives us this, ERA is considered to be an excellent metric for evaluating pitchers. On average, how many runs does a pitcher allow that are his fault in a given game?
ERA is not a perfect metrics because many different factors can affect it.
Henry Chadwick, statistician and writer, invented ERA in the mid-to-late 19th century. At the time, the win-loss record was the primary metric used to evaluate pitchers. Chadwick believed this metric was insufficient, and create ERA.