Earned run average represents the number of earned runs a pitcher allows per nine innings -- with earned runs being any runs that scored without the aid of an error or a passed ball. ERA is the most commonly accepted statistical tool for evaluating pitchers.
The formula for finding ERA is: 9 x earned runs / innings pitched. If a pitcher exits a game with runners on base, any earned runs scored by those runners will count against him.
ERA should be an ideal evaluation of pitchers. The goal of pitching is to prevent runs from scoring, and ERA tells us basically how well a pitcher does that. How many runs does he allow, on average, that are his fault in a given game?
But there are a few flaws with ERA, because so many different factors can affect it. While defensive mistakes are taken into account, great defensive plays are not. So a pitcher with an average defense is at a disadvantage to a pitcher with a great defense. It's also hard to evaluate ERA across the two leagues in Major League Baseball, because the absence of a designated hitter in the National League tends to keep pitchers' ERAs lower. Even the ballpark in which a pitcher pitches can affect a pitcher's ERA because certain stadiums are more conducive to run scoring.
Still, ERA is a fantastic tool for measuring a starting pitcher's success. However, it's not quite as effective in measuring relief pitchers, who often pitch only fractions of an inning -- sometimes leaving their ERA in the hands of other relievers. Even relief pitchers who pitch a full inning tend to exert all their energy on those three outs, instead of spreading it out over the course of a game. This means relievers generally have lower ERAs than starting pitchers.
Statistician and writer Henry Chadwick gets credit for inventing ERA in the mid-to-late 19th century. His thinking was that win-loss record simply didn't go far enough in determining the mark of a good pitcher. The statistic caught on in the 20th century, when relief pitchers became more prevalent. This made win-loss records even less reliable, because a starting pitcher could pitch a great game and not receive credit in the win column if his bullpen didn't preserve the victory.
Watch: Jeremy Guthrie allows a home run, causing his earned run average to rise.
In A Call
ERA is generally referred to directly after an announcer gives a pitcher's win total. Something like: "He's 2-0 with a 3.33 ERA."