Adjusted ERA+

Adjusted ERA+, often simply abbreviated to ERA+ or ERA plus, is a pitching statistic in baseball. It adjusts a pitcher's earned run average (ERA) according to the pitcher's ballpark (in case the ballpark favors batters or pitchers) and the ERA of the pitcher's league.

Formula

Baseball-Reference.com originally calculated it as

$ERA+=100*{lgERA \over ERA}*PF$

Where ERA is the pitcher's ERA, lgERA is the average ERA of the league, and PF is the park factor of the pitcher in question.

Today, they calculate it as

$ERA+=100*(2-{ERA \over lgERA}*{1 \over PF})$ ^[1]

The average ERA+ is set to be 100; a score above 100 indicates that the pitcher performed better than average, while below 100 indicates worse than average. For instance, imagine the average ERA in the league is 4.00: if pitcher A has an ERA of 4.00 but is pitching in a ballpark that favors hitters, his ERA+ will be over 100. Likewise, if pitcher B has an ERA of 4.00 but pitches in a ballpark favoring pitchers, then his ERA+ will be below 100.

As a result, ERA+ can be used to compare pitchers across different run environments. In the above example, while ERA will lead one to believe that both pitchers pitched at the same level due to their ERAs being equivalent, ERA+ indicates that pitcher A performed better than pitcher B. ERA+ can be used to account for this misleading impression.

Leaders

Pedro Martínez holds the modern record for highest ERA+ in a single season; he posted a 1.74 ERA in the 2000 American League, which had an average ERA of 4.92, which gave Martínez an ERA+ of 291.^[2] While Bob Gibson has the lowest ERA in modern times (1.12 in the National League in 1968), the average ERA was 2.99 that year (the so-called Year of the Pitcher) and so Gibson's ERA+ is 258, sixth highest since 1900. 1968 was the last year that Major League Baseball employed the use of a pitcher's mound greater than 10 inches.^[3]

The career record for ERA+ (with a minimum of 1,000 innings pitched) is held by Mariano Rivera, a closer whose career ERA+ is 205. Upon retirement in 2013, with an ERA+ of 194 in his final season, Rivera's career record of 205 surpassed the record among retired players of 154, held by Martínez, bumping Jim Devlin, a pitcher in the 1870s, to third with 151.^[4] Among qualifying pitchers, Pedro Martínez has the most separate seasons with an ERA+ over 200, with five, and the most consecutive 200 ERA+ seasons (4), though the closer Rivera, with too few innings each year to qualify officially, has surpassed 200 ERA+ in 13 seasons of his 19 seasons, including 4 consecutive seasons twice and 5 consecutive seasons once and also surpassing 300 in 2004 and again in 2008. Roger Clemens topped a 200 ERA+ three times, and Greg Maddux had two such seasons.

Players in bold are active as of the middle of the 2017 season and have not announced their retirement.

Single-season leaders include only pitchers eligible for the ERA title (a pitcher must throw a minimum of one inning per game scheduled for his team during the season to qualify for the ERA title).^[5] Only pitchers with 1,000 or more innings pitched are shown in the career leader list.

Career Leaders
Rank	Player	Adjusted ERA+
1	Mariano Rivera	205
2	Clayton Kershaw	159
3	Pedro Martínez	154
4	Jim Devlin	150
5	Lefty Grove	148
T-6	Walter Johnson	147
T-6	Hoyt Wilhelm	147
T-8	Dan Quisenberry	146
T-8	Smoky Joe Wood	146
10	Ed Walsh	145
11	Roger Clemens	143
T-12	Addie Joss	142
T-12	Brandon Webb	142
14	Trevor Hoffman	141
15	Kid Nichols	140
16	Mordecai Brown	139
T-17	John Franco	138
	Chris Sale
	Cy Young
T-20	Johan Santana	136
T-20	Bruce Sutter	136
T-22	Pete Alexander	135
	Christy Mathewson
	Randy Johnson
	Rube Waddell

Single Season Leaders
Rank	Player	Adjusted ERA+	Year	Team
1	Tim Keefe	293	1880	Troy Trojans
2	Pedro Martínez	291	2000	Boston Red Sox
3	Dutch Leonard	282	1914	Boston Red Sox
4	Greg Maddux	271	1994	Atlanta Braves
5	Greg Maddux	260	1995	Atlanta Braves
6	Walter Johnson	259	1913	Washington Senators
7	Bob Gibson	258	1968	St. Louis Cardinals
8	Mordecai Brown	253	1906	Chicago Cubs
9	Pedro Martínez	243	1999	Boston Red Sox
10	Walter Johnson	240	1912	Washington Senators
11	Christy Mathewson	230	1905	New York Giants
12	Dwight Gooden	229	1985	New York Mets
13	Roger Clemens	226	2005	Houston Astros
14	Pete Alexander	225	1915	Philadelphia Phillies
T-15	Christy Mathewson	222	1909	New York Giants
T-15	Roger Clemens	222	1997	Toronto Blue Jays
T-17	Cy Young	219	1901	Boston Americans
T-17	Pedro Martínez	219	1997	Montreal Expos
19	Denny Driscoll	218	1882	Pittsburgh Alleghenys
20	Lefty Grove	217	1931	Philadelphia Athletics

References

↑ Forman, Sean. "We Goofed: ERA+ numbers » Baseball-Reference Blog » Blog Archive". www.baseball-reference.com. Retrieved 1 August 2016.
↑ Single season awards require a minimum of one inning pitched for each game played (thus usually 162 IP in today's game). Thus closers and other relievers will not generally acquire enough innings pitched to qualify.
↑ Baseball Trivia (General) - Pitchers mound, allexperts.com.
↑ Baseball Reference career ERA+ leaders, accessed October 21, 2013 (after the 2013 regular season, with Rivera having pitched his last game, had his number retired by the Yankees, but Baseball Reference not yet having marked him as retired on their site). Also the source for the corresponding table.
↑ "Official Rules: 10.00 The Official Scorer". Major League Baseball. Retrieved 15 October 2016.

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