This paper examines and analyzes the average ERA in major-league baseball each season between 1871 and 2019. The data shows that the maximum average ERA of 5.33 occurred in 1894 after the pitching distance was increased to 60 feet 6 inches in 1893. The lowest average ERA of 2.19 occurred in 1874 and the overall average ERA across baseball history is 3.74. From a current perspective, the overall average ERA of almost exactly 4.0 since 1920 is a more useful benchmark given the significant changes that were taking place as the game evolved over its first fifty years.
The data is used to divide baseball history into different pitching eras based on the similarity of average ERA across different time periods. For example, the overall average ERA for the years 1921-1928 was 4.05. This era is designated the Most of the Twenties Era. The lowest overall average ERA of 2.82 occurred during the appropriately named Deadball Era (1904-1919). Not surprisingly, the offensive explosion that occurred during the 1990s shows up in the average ERA data. The overall average ERA between 1994 and 2009 (designated the Camden Yards Era) was the highest for any era in baseball history, 4.46. In terms of understanding how pitching has evolved, these data driven pitching era designations are an improvement over other ways of dividing baseball history because the variation in average ERA over the time periods (measured using standard deviation) is smaller than the variation in average ERA during traditional historic eras.