In baseball statistics, earned run average (ERA) is the mean of earned runs given up by a pitcher per nine innings pitched. It is determined by multiplying the number of earned runs allowed by nine and dividing by the number of innings pitched. Runners reaching base on errors (even errors by pitchers) do not count toward ERA if they later score. Some sources may list players with undefined or infinite career ERAs. This can happen if a pitcher allows one or more runs without retiring a batter (usually in a single appearance).