ERA stands for Earned Run Average, which is a statistic used in baseball to measure a pitchers effectiveness. It represents the number of earned runs a pitcher allows per nine innings pitched, with earned runs being any runs that scored without the aid of an error or a passed ball. The formula for finding ERA is 9 x earned runs / innings pitched. A lower ERA is better, and it is the most commonly accepted statistical tool for evaluating pitchers. However, 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. Its 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.