Earned Run Average

Where:

ER = Earned Runs
IP = Innings Pitched
9 is used because that is the standard amount of innings in a regulation game

Earned Run Average (ERA) is a formula used in baseball to evaluate the performance of a pitcher by tracking the average amount of runs he allows in a nine inning game. Like a person's batting average is used to evaluate how skillful a batter a person is, ERA is used to calculate how skillful a pitcher is.

To calculate a pitcher's ERA, simply divide the amount of runs he/she gave up by the amount of innings he/she pitched and multiply that number by nine.

What's the Nine For?
People are able to judge how skillful a pitcher based on his/her ERA by knowing how many runs he/she allows every inning of a standard game. Therefore, when calculating one's ERA you must multiply by nine since that is the standard amount of innings played in a regulation game.

How Did The Equation For ERA Come About?
Originally, pitchers pitched entire games and the effectiveness of their pitching was measured by how many games their team won. However, in the 1900s, relief pitchers became more widely used.

This meant that tabulating the amount of games a pitcher's team won was no longer sufficient for measuring his skills. That's when Henry Chadwick saved the day by creating the modern ERA equation.

Today, a person's ERA is considered "very good" when it's 4.00 or less. This means that a good pitcher allows merely 4 runs to be scored in nine innings of pitching. As was mentioned before, it is very unlikely that a pitcher will pitch a whole nine-inning game so that's where the 9 in the equation comes in; even though a pitcher may have only pitched 4 innings of a game, his ERA is based on nine innings of pitching.

What's considered a good ERA changes on a yearly basis. Here are some comparative figures:

An ERA of:	was considered very good in:
Below 2.00	1910
Below 4.00	1920
Below 4.00	1930
Below 4.00	1940
Below 4.00	1950
Below 2.00	1960
Below 2.00	1970
Below 2.00	1980
Below 2.00	1990
Below 4.00	2000

All-time lowest ERA in one season of pitching: 0.86 set by Tim Keefe in 1880.
Modern lowest ERA: 1.12 by Bob Gibson in 1968
Highest ERA: infinity (twenty pitchers in Major League history)

How Can an ERA Be Infinity?
If the amount of innings the pitcher pitched is zero his/her ERA will be infinity.

For example, say a pitcher goes to the mound at the beginning of the 1st inning and allows one or more runs to be scored before the first out is made. His coach then sends in another pitcher because he's not satisfied with the starting pitcher's performance. According to ERA calculation, the pitcher did not pitch any innings in that game.

Say the he had allowed to 2 runs to be scored before he was taken out. His ERA for that game would be 2/0 * 9, which, when solved, is 0.; Since it's not zero because he didn't allow any runs to be scored (if he hadn't allowed any runs and made at least one out, his ERA would be 0 which is the best anyone can get,) you can't say his ERA is 0, so you must say it's infinity.

The reason the above situation only works if no outs had been made is because an out counts as 1/3 of an inning and would give an actual number for the ERA.

Misleading Leaders
A relief pitcher's ERA does not always accurately convey his/her skills (or lack thereof.) For example, say a relief pitcher starts to pitch in the 6th inning with 2 outs and the bases loaded. At that point in the game, his team is winning by one run. If the batter against him hits a single and the opposing team scores two runs, he is not said to be responsible for those runs since the pitcher before him caused the bases to be loaded. If he strikes the next batter out, his ERA for the game is 0.00 although he very well may have caused his team to lose.

For starting pitchers, ERA is a little more accurate but there are still some slim cases in which it cannot be trusted to evaluate a starting pitcher's true performance. For example, situations in which there is very poor weather or that the altitude of the stadium is less than ideal may cause the pitcher to seem less talented than he/she actually is.

Very strong winds can affect the path of the ball (before and after it reaches the batter,) rain and snow can inhibit a pitcher from focusing on his target clearly, and high altitudes (like that of Denver where the Colorado Rockies play) affect a pitcher's ability to throw breaking balls. High altitudes also allow fly balls to soar 10 % farther and higher than at sea level.

A Partial Solution to "Unfair" ERAs: DIPS
Many things that pitchers can't control affect their ERA. For instance, if a batter scores a run because an infielder or outfielder drops a ball or overthrows, the run still counts against the pitcher's ERA.

Since quite a few people do not find this very fair, a new formula was created that only focuses on the plays/runs that do not have to do with infielders. This formula deals only with the runs scored because of walks, strikeouts, hit batters, and home runs. It is referred to as Defensive Independent Pitching Statistics (DIPS.)

This one is quite more complex than ERA:

Where:
*HR= home runs *BB= walks *HBP= hit batters *K= strikeouts *IP = innings pitched

Example:
Eddie is a pitcher for a high school baseball team called the Eagles. He wants to calculate his ERA for the year so far. His team has played 7 games and each game lasts 9 innings. He usually pitches for the first 6 innings of each game. To make things easy, let';s say that within the first 6 innings of every game the Eagles have played, the opposing team has scored 3 runs.

According to Order of Operations, we start with the multiplication:

Eddie's ERA is 4.5 which means that if he were to pitch for all nine innings of every game, he'd allow an average of 4.5 runs to be scored against his team.

Helpful Definitions:
Breaking Balls: Pitched balls that move more awkwardly and slower than fastballs as to confuse the batter. The following are the types of breaking balls and their definitions:

Curveball: A ball thrown with a special grip and motion so that it curves in an exaggerated fashion

Slider/Nickel Curve: A pitch thrown faster and with more drop than a curveball.

Screwball: A ball that curves in the opposite direction of a curveball.

Related Websites:

http://ask.metafilter.com/mefi/18366

http://en.wikipedia.org/wiki/Earned_run_average

http://en.wikipedia.org/wiki/Defense_independent_pitching_statistics

http://www.freemathhelp.com/earned-run-average.html

Works Cited:
"Defense Independent Pitching Statistics." Wikipedia.com. 23 May 2006. Wikimedia
Foundation. 9 June 2006.
<http://en.wikipedia.org/wiki/Defense_independent_pitching_statistics>.

"Earned Run Average."Wikipedia.com. 23 May 2006. Wikimedia
Foundation. 9 June 2006. <http://en.wikipedia.org/wiki/Earned_run_average>.

"1940 Earned Run Average Leaders."ESPN.com. 2006. ESPN Internet
Ventures. 9 June 2006. <http://sports.espn.go.com/mlb/alltime/leaders?type=1&sort=1&breakdown=0&year=1940>.

"1950 Earned Run Average Leaders." Baseball_almanac.com. 2000. The
Top 25: A Leaderboard. 9 June 2006.
<http://www.baseball-almanac.com/yearly/top25.php?s=ERA&l=AL&y=1950>.

"1970 Earned Run Average Leaders." Baseball_almancac.com. 2000.
The Top 25: A Leaderboard. 9 June 2006.
<http://www.baseball-almanac.com/yearly/top25.php?s=ERA&l=AL&y=1970>.

"1990 Earned Run Average Leaders." ESPN.com. 2006. ESPN Internet
Ventures. 9 June 2006. <http://sports.espn.go.com/mlb/alltime/leaders?type=1&sort=1&breakdown=0&year=1990>.

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