Earned run average

Prior to 1900 and for many years afterward, pitchers were routinely expected to pitch a complete game, and their win–loss record was considered sufficient in determining their effectiveness.

[2] The all-time record for the lowest single season earned run average by a pitcher pitching 300 or more innings is 1.12, set by Bob Gibson in 1968.

The record for the lowest career earned run average is 1.82, held by Ed Walsh, who played from 1904 through 1917.

Outfielder Vic Davalillo made two pitching appearances with St. Louis Cardinals in 1969 and failed to record an out while facing four batters and allowing one run, leaving him with an infinite ERA as a major-league pitcher.

At times it can be misleading to judge relief pitchers solely on ERA, because they are charged only for runs scored by batters who reached base while batting against them.

The advent of the designated hitter rule in the American League in 1973 made the pitching environment significantly different.

The combination of high altitude (5,280 ft or 1,610 m) and a semi-arid climate in Denver causes fly balls to travel up to 10% further than at sea level.

Denver's altitude and low humidity also diminish the ability of pitchers to throw effective breaking balls, due to both reduced air resistance and difficulty in gripping dry baseballs.

Indeed, hometown hurlers have some advantage in any given game since they are physically acclimated to the altitude and often develop techniques to mitigate the challenges of this ballpark.

In modern baseball, sabermetrics uses several defense independent pitching statistics (DIPS) including a Defense-Independent ERA in an attempt to measure a pitcher's ability regardless of factors outside his control.

[3] Silver also developed a "quick" earned run average (QuikERA or QERA) to calculate an ERA from peripheral statistics including strikeouts, walks, and groundball percentage.

Because of rule changes post-1920, most notably the abolition of the spitball and frequent replacement of soiled or scuffed baseballs, the increased importance of the home run (largely due to Babe Ruth), and the American League's adoption of the designated hitter rule, ERAs have been noticeably higher than in the early decades of the sport.