This is the number of earned runs a pitcher allows multiplied by 9 and then divided by the number of innings pitched. The number of earned runs is multiplied by 9 since there are 9 innings in a standard Major League Baseball game. The formula is fairly easy to apply. For example, if Randy Johnson gives up 3 earned runs in 7 innings, his ERA for that game would be: [3 (earned runs) x 9]/ 7 (innings pitched) = 3.86.
A few things you should know about ERA. If a run scores as a result of an error, it does not count against a pitcher’s ERA. Hence the “E” in ERA stands for “earned”. Also, while it is often assumed to be the “holy grail” of baseball metrics to evaluate a pitcher (WHIP being a close second), using ERA in and of itself does have drawbacks. First, hits allowed (which in turn lead to earned runs) are often out of the pitcher’s control. How many times, for example, have we seen a pitcher get saved by an outstanding diving play from the third baseman, while another pitcher get robbed by a broken bat bloop single or a less than stellar effort from his defense? The point being is these fluke bloop singles and other “out of the pitchers’ hands” scenarios can of course lead to “earned runs” and a higher ERA.
We should also note that American League Pitchers will generally have a higher ERA than National League Pitchers given the AL has a designated hitter. The designated hitter rule will almost always inflate the American League metrics for both pitching and hitting.
So as a frame of reference, here is the Average ERA in MLB, MedianERA in MLB, Average ERA in the NL and Average ERA in the NL. All of these figures are as of June 1, 2004.
MLB Average 4.37 ERA
MLB Median 4.40 ERA
NL Average 4.17 ERA
AL Average 4.61 ERA
So for evaluation purposes, an ERA below 4.37, would be considered relatively good compared to the league average as a whole. The lower it goes, the stronger the ERA becomes. The reverse, obviously, holds true as well. The higher the ERA, the weaker.
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