Joe Pagano

The Earned Run Average (ERA) In Baseball - Nothing More Than a Complex Fraction



Posted: Sunday, May 08, 2011

by Joe Pagano
Math by Joe

Now that baseball season is well under way, many of you might be wondering how that statistic called the earned run average (era) is computed. You might know that this stat reflects a pitcher's allowed, or earned, runs per nine innings. But did you know that this stat is nothing more than a complex fraction in mathematics and can be calculated with a nice little trick?

The earned run average can be a pitcher's best friend or worst nightmare. Regardless of how this stat is actually calculated, the lower the number the better for the pitcher. Indeed a pitcher that can end the season with an ERA of under 2, would be very pleased, provided the pitcher threw at least 50 or more innings. A pitcher with few appearances could have the ERA work very favorably if he did not allow any runs; while a pitcher who threw for 1 inning, yet allowed 10 runs, would have a disastrous ERA.

Yet how do we get to this calculation and what does this have to do with complex fractions? A complex fraction you might recall, is a fraction which contains in either the numerator, the denominator, or both, another fraction. This is why it is considered complex. The earned run average in baseball is computed by taking the total of earned runs and dividing that by the number of innings pitched divided by nine. That "double division" in the last sentence is where our complex fraction comes in.

Let's see this calculation with an example. Suppose Mariano Rivera of the New York Yankees, has pitched 72 innings. Let's also assume that during these innings he has allowed 6 earned runs. The way to get his ERA is as follows: we divide 6 by 72 divided by 9 or ERA = 6/(72/9). Since 72/9 is 8, the calculation simplifies to ERA = 6/8 or 0.75; not too shabby an earned run average. In this calculation, we performed the 72/9 calculation first but we could use the principle that dividing is the same as multiplying by the reciprocal. This is a nice little trick to getting the ERA.

The way we do this is as follows: we convert 6/(72/9) into 6*(9/72) which becomes 54/72, and this simplifies to 3/4, or 0.75. Thus to get the ERA quickly, take the earned runs and multiply them by 9; then divide by the number of innings pitched. To see this, suppose Johann Santana has given up 18 earned runs in 100 innings pitched. His ERA will be 18*9/100 or 162/100 or 1.62. Now that you are aware of this neat little way to get the ERA, you can show your friends what a real baseball fan you are.
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