For as long as can be remembered, ERA has been considered to be a good measure of the success of a pitcher. But unfortunately it really can be a misleading metric.
Earned run average is calculated in the amount of earned runs per 9 innings pitched.
ERA = ER * 9 / IP
The fallacy with ERA lies in the theory that a pitcher controls everything involved with the statistic. The definition of an earned run is a run scored that is not as a result of an error. Unearned runs, which are not counted in ERA, are based on whether a player in the field was charged with an error or not. There really isn’t a particularly consistent guideline for what is an error, either. It is one of those “I know it when I see it” things unfortunately.
Of larger concern however, is the theory that unearned runs don’t matter. While an unearned run may not be the pitcher’s fault, it is not a particularly fair judge of a pitcher either. Since ERA only measures the Earned Runs and not other stats such as hits or walks, a pitcher can very easily post a low ERA and not pitch particularly effectively.