Earned Run Average (ERA)
From SABR Encyclopedia
In baseball statistics, Earned Run Average or ERA is a measure of how many earned runs a pitcher can be expected to give up in nine full innings. Earned Run Average is an official MLB statistic. Earned Run Average is shown with two decimal places. Earned Run Average is calculated using this formula from rule 10.21(e) of the 2009 Official Rules[1]:
A pitcher qualifies if he has pitched at least one inning for every scheduled game. Pitching championship qualification is found in rule 10.22(b) of the 2009 Official Rules[2]
[edit] History
The current qualifications were put in the rule book in 1951 [3]. Prior to 1951, the qualification was at least 10 complete games (CG) and 100 innings pitched "but men who met the 1951 standard were usually recognized as leaders" with less than 10 CGs [3].
Innings pitched (IP) has the unique status of being the only official counting statistic that is not an integer. Instead, it is counted in thirds. Until 1978, season IPs for each pitcher were rounded off before the pitcher's ERA was calculated. This meant that a pitcher whose season IP was 1/3 (often represented as 0.1) would not be rounded down to 0 IP before calculating his ERA. If this sip of coffee pitcher had allowed 1 ER, before 1978 his ERA was infinity but starting in 1978 his ERA would be 27.00. In 1981 the ERA championship was ultra competitive. Using the usual rounding method, Steve McCatty's ERA was 2.32 ((9*48 ER)/186 IP) to nip Sammy Stewart's 2.33 ((9*29 ER)/112 IP). But if one didn't round the innings off, McCatty's ERA was 2.33 ((9*48 ER)/185 2/3 IP) above Sammy Stewart's 2.32 ((9*29 ER)/112 1/3 IP). McCatty was given the ERA title, but the 1982 rule book added the statement that for the calculation of ERAs, fractional innings were not to be rounded off[3]. It is interesting that if you query www.mlb.com for the ERA leaders for the AL in 1981, Stewart is listed first. [4]. Similarly, current databases store fractional IP and calculate ERA on demand using current no-rounding rule, so IP and ERA from electronic sources on some cases will be slightly different than IP and ERA from contemporaneous printed sources.
Footnotes
- ↑ Official Rules of Major League Baseball, rule 10.21(e).
- ↑ Official Rules of Major League Baseball, rule 10.22(b).
- ↑ 3.0 3.1 3.2 The Baseball Encyclopedia 9th edition. New York: Macmillan, 1993.))
- ↑ http://mlb.mlb.com/stats/historical/leaders.jsp?c_id=mlb&baseballScope=AL&statType=2&sortByStat=ERA&timeSubFrame2=1&timeFrame=1&timeSubFrame=1981, retrieved on 2009-10-24.
