Author: Douglas J. Jordan1

1Department of Business Administration, Sonoma State University, Rohnert Park, CA, USA

Corresponding Author:
Douglas J. Jordan
3663 Primrose Avenue
Santa Rosa, CA 95407
jordand@sonoma.edu
707-206-0563

Douglas J. Jordan, PhD, is a Professor of Business Administration at Sonoma State University in Rohnert Park, California. In addition to his professional interest in corporate finance and investments, he is a member of the Society for American Baseball Research and does research on baseball related topics.

Eras of ERA

ABSTRACT

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.

Key Words: Major League Baseball, Earned run average

INTRODUCTION

Baseball-Reference.com has seasonal data for major league baseball going back to 1871. The web site credits Pete Palmer and Gary Gillette with generating this data set. It includes both batting and pitching statistics on a per team game basis. All this data can be exported to Excel and then easily plotted. The data set includes the annual average earned run average (ERA) for major-league baseball going back to 1871. This average ERA data is examined and analyzed to better understand how pitching has evolved over the course of baseball history. The results show that the era designations typically used to describe baseball history do not adequately describe ERA changes since 1871. This article suggests alternative time frames and designations for pitching eras in baseball history.

Rules Changes That Have Affected Pitching

Striking the proper balance between pitching and offence is a perennial theme in baseball history. Too few runs being scored, or too many, makes the game unattractive to both players and spectators. This has led the ruling bodies of baseball to modify pitching rules over time in an attempt to maintain the delicate balance between pitching and hitting. Some of the more important rules changes are identified and listed below.

  • All restrictions lifted on pitching motion in 1884 (14).
  • Pitching distance increased from 50 feet to 60 feet 6 inches and pitching rubber installed in 1893 (2).
  • New ball with a center of cork rather than rubber in 1911(7,11,16).
  • Ball constructed using Australian yarn in 1919 (13,16).
  • Spitball outlawed and use of cleaner balls in 1920 and 1921 (17,18).
  • “Cushion cork” centered ball and raised seams in 1931 (8,17).
  • Over 500 MLB players gone during WWII and “baleta” ball in 1942-1946 (3,15).
  • Strike zone redefined in 1963 (10).
  • Mound lowered and redefinition of strike zone in 1969 (1).
  • Rawlings becomes official supplier of balls in 1977 (4,12).
  • Early 1990s increase in offense in 1993 and 1994 Multiple possible reasons include steroids, expansion, new ballparks, and changes to the ball (5,17).

Not surprisingly, when the pitching rules are changed, there is an effect on the average ERA in baseball. The relationship between rules changes and average ERA shows up clearly in Figure 1. The data in Figure 1 also provides the framework to compare traditional designations of eras in baseball history with data driven pitching eras in accordance with the results shown in Figure 1. The purpose of this paper is to derive data driven pitching eras that better represent baseball history from a pitching perspective than the traditional era designations.

METHODS

Palmer and Gillette have done baseball researchers an invaluable service by generating major league baseball batting and pitching year-by-year average data going back to 1871. Most of the data (for example, runs/game, hits, and home runs) is presented on a per team game basis. However, the data for ERA is simply the average ERA across baseball that year. For this analysis, the ERA data from 1871 – 2019 is exported to Excel and then plotted. The raw data is shown in Figure 1 along with arrows indicating when the rules changes that impacted pitching were implemented.

Although the data in Figure 1 is interesting in itself, the primary purpose of this analysis is to argue that conventional designations of different eras in baseball history do not conform well to what was happening in the game from a pitching perspective. This makes it necessary to compare the conventional era designations to the era designations proposed in this paper. The comparison is done by looking at the average of the average ERA over each era for both the conventional eras and the proposed eras. This average of average ERA is then used to calculate the standard deviation of average ERA over the era. A smaller standard deviation for a given era indicates that the data better represents what was happening during that time from a pitching perspective. Figure 2 shows conventional era designations superimposed on the raw ERA data. Figure 3 shows the proposed era designations superimposed on the original data and Table 1 gives the results of the standard deviation calculations for those eras. 

RESULTS

Plotting the Raw Data

Figure 1 shows the Palmer and Gillette data for ERA going back to 1871. In addition to the raw data, the figure also points out some specific years when pitching rule changes occurred.

Figure 1: MLB Average ERA since 1871 with years of significant pitching rules changes identified.

Figure 1

Conventional Era Designations

Since the purpose of this paper is to divide baseball history into periods that better describe pitching eras, it is necessary to compare the results to traditionally defined eras. Bill James has developed a baseball history timeline where he establishes baseball eras through the history of the game. The James eras are going to be the baseline against which the data generated eras are compared because James uses a quantifiable methodology to determine his eras. That methodology is described in detail in Fools Rush Inn (9). Alternative definitions of baseball eras are more subjective. The average ERA for each of James’ eras is shown in Figure 2.

Figure 2: Average ERA data shown with Bill James’ era designations

Figure 2

Data Driven Pitching Eras

The problem with using established era designations is that these eras do not necessarily conform to what was happening in baseball at the time from a pitching perspective. Therefore, the range of average ERA is so wide for most of these eras. But if we want to better understand baseball history from a pitching perspective, we can let the average ERA data speak for itself. In other words, we can choose eras based on when the average ERA data in Figure 1 was relatively constant over time. This will give us a better understanding of how average ERA changed over time. The results of doing this are shown in Figure 3. 

Figure 3: Average ERA data shown with data defined pitching era designations

Figure 3

Measuring the Variation in ERA During Different Eras

Now we get to the heart of the question. Are the data driven pitching era designations an improvement over the Bill James era designations? If so, why? The answer is yes, the ERA data driven designations are better because the variation in ERA over these eras is smaller than the variation in ERA over the James era designations. That means that the average ERA better represents the pitching data during the era because the standard deviations are lower. This assertion can be tested by looking at the standard deviation of the average ERA over different eras since standard deviation is a statistical measure of the amount of dispersion in a set of data. Higher standard deviations indicate greater dispersion of the data while lower standard deviation means lower dispersion. The results of standard deviation calculations for the each of the eras shown in Figures 2 and 3 are given in Table 1  

Table 1: Average ERA and Standard Deviation of Average ERA for Designated Eras

Bill James’ Era Designations Data Driven Pitching Era Designations
Years Era Name Average
ERA		 Std. Dev.
of ERA		 Years Era Name Average
ERA		 Std. Dev.
of ERA
1871-1892 Pioneer Era 3.09 0.635 1871-1892 Pioneer Era 3.09 0.635
1893-1919 Spitball Era 3.31 0.778 1893-1903 60’6" Transition Era 4.03 0.709
1920-1946 Landis Era 4.02 0.364 1904-1919 Deadball Era 2.82 0.284
1947-1968 Baby Boomer Era 3.82 0.318 1921-1928 Most of the Twenties Era 4.05 0.12
1969-1992 Artificial Turf 3.77 0.210 1931-1941 Mostly the Thirties Era 4.18 0.20
1993-2012 Camden Yards Era 4.31 0.254 1942-1946 World War II Era 3.47 0.093
  Overall Average 3.74 0.624 1947-1962 Mostly the Fifties Era 3.98 0.170
  1963-1968 The Low Sixties Era 3.39 0.222
1973-1992 Mostly the Seventies and Eighties Era 3.82 0.174
1994-2009 Camden Yards Era 4.46 0.14

DISCUSSION

Figure 1 shows that average annual ERA has varied dramatically since 1871. The biggest changes occurred in the early years of the game when the rules were evolving quickly (often annually). The lowest average ERA of 2.19 occurred in 1874 and the highest average ERA of 5.33 happened exactly 20 years later in 1894 (when the pitching distance was increased to 60 feet 6 inches). The overall average for the 149 years in the data set is 3.74. However, given the dramatic changes in average ERA in the data set (especially in the first fifty years), this overall average ERA is not of much practical use. A more useful single number to characterize baseball’s ERA over time is the average ERA since 1920. That number is almost exactly 4.0 (3.99 to be precise). This is the best single number to remember in terms of average ERA over time. Although, as we shall see, the data falls nicely into shorter time periods with average ERAs higher or lower than 4.0. 

Figure 1 also shows that most of the rule changes had a significant effect on average ERA. For example, average ERA increased from 3.28 in 1892 to 4.66 in 1893 when the pitching distance increased to 60 feet 6 inches and the pitching rubber was installed. But pitchers were able to adjust to being further away and average ERA was down to 3.60 by 1898. More recently (relatively speaking) after the “Year of the Pitcher” in 1968, baseball lowered the pitching mound and redefined the strike zone to increase offense (19). It worked. Average ERA went from 2.98 in 1968 to 3.61 in 1969. It is not necessary to verbally describe all the changes shown in Figure 1 here. The important point is that rules changes do result in changes to average ERA. This may seem obvious, but the data in Figure 1 provides empirical evidence for the assertion that rules changes can impact pitching in general and the average ERA.

Dividing Baseball History into Eras

Baseball’s long history is often divided into shorter timeframes (or eras) to better understand how the game has changed over time. But there is no agreed upon standard for what those eras are or what they should be called. For example, Woltring, Rost, and Jubenville use the Deadball Era (1901-1919), the Live Ball Era (1920-1941), the Integration Era (1942-1960), the Expansion Era (1961-1976), the Free Agency Era (1977-1993), the Long Ball/Steroid Era (1994-2005), and the Post Steroid Era (2006-2011) in their statistical examination of On-Base Plus Slugging Percentage (20). But Bill James divides baseball history into a completely different set of eras in his book Fools Rush Inn (9). James uses the Pioneer Era (1871-1892), the Spitball Era (1893-1919), the Landis Era (1920-1946), the Baby Boomers Era (1947-1968), the Artificial Turf Era (1969-1992), and the Camden Yards Era (1993-2012). There is no real right or wrong way to divide the history, but these two examples show that there can be substantial differences in eras depending on who is doing the dividing.

Figure 2 shows the ERA data for the entire period, with the Bill James eras and the average ERA for each era superimposed on the original data. The average ERAs during the Pioneer Era and the Spitball Era were 3.09 and 3.31, respectively. These two numbers are the lowest average ERAs for James’ eras. However, from a practical perspective, these average ERAs are not especially useful because of the tremendous variation in ERA during each time. In the Spitball Era, the highest average ERA was 5.33 (in 1894) and the lowest was 2.37 (in 1908). This means that the average of 3.31 does not tell you very much about pitching ERA during that period. It is the correct mathematical average, but it’s not very helpful because of the range of ERAs in the era.

The wide range of ERAs during these two time periods is a result of the relatively rapid evolution of the game during these two time periods. But by roughly 1920, the game had matured to the point that the variation in average ERA declined significantly. The average ERA increased to 4.02 during the Landis Era. That number is informative. It tells you the average ERA during the Twenties and Thirties was about 4.0. There’s still substantial variation, average ERA was as high as 4.81 in 1931 and as low as 3.81 in 1933, but the average of 4.0 does provide a good benchmark ERA for the time.

Average ERA declined to about 3.8 during the Baby Boomer Era (average ERA was 3.82) and the Artificial Turf Era (average ERA was 3.77) but the range of average ERA was wider during the Baby Boomer Era. The peak ERA during that era of 4.36 occurred in 1950 while the low occurred during the Year of the Pitcher in 1968 (average ERA was 2.98). The highest average ERA during the Artificial Turf Era was 4.29 in 1987 while the lowest average ERA of 3.26 occurred in 1972. The boom in offense that took place in the early 1990s caused average ERA to increase substantially between 1992 and the year 2000. The average ERA for the whole Camden Yards Era was 4.31 with the highest average of 4.77 occurring in 2000 and the low of 3.74 happening in 2014.

Discussion of the Data Driven Pitching Eras in Figure 3

The rapidly evolving rules during the Pioneer Era led to large variations in average ERA during this era. The data itself does not suggest a better way to characterize pitching during this period so the designation Pioneer Era is retained. However, the change to a 60 feet 6-inch pitching distance in 1893 had a tremendous effect on average ERA for two years before pitchers adjusted to the new distance. As pitchers got used to pitching from further away after 1894, the average ERA declined in an almost monotonically between 1894 and 1904. This precipitous decline in average ERA makes an average ERA for that period misleading. Since there is nothing to be learned by calculating the average ERA in this time, no average will be given. Therefore, the period 1893 – 1903 is referred to as the 60 feet 6-inch Transition Period. Other periods or years will also be designated as transitionary periods if the average ERAs for the period do not match well with the previous or following eras.

In accordance with common usage, the next era is called the Deadball Era. Following the transitionary period, the Deadball Era refers to the years 1904 – 1919. The name is appropriate for the era. Average ERA during the Deadball Era was the lowest in baseball history, 2.82. Average ERA during this period would have been even lower if not for a brief surge in offense in 1911 and 1912 due to cork being inserted into the center of the ball (7,11,16). But once again, pitchers adjusted, and average ERA declined to about 2.75 within a few years. John McMurray of the Society for American Baseball Research argued that the reason for the post-1912 decline in average ERA was pitchers scuffing the ball (6).

The Twenties and Thirties

Average ERA increased from 3.07 in 1919 to 4.05 in 1921 ushering in what is often referred to as the live-ball era and what Bill James calls the Landis Era. Figure 3 clearly shows that the average ERA of 3.46 in 1920 does not conform to average ERA for the periods before or after that year. So rather than trying to force 1920 into a period it does not belong in, it can be recognized that 1920 was another transitionary year and leave it out of the eras that preceded or followed it.

With the exception of a small bump in average ERA during 1925, average ERA between 1921 and 1929 was consistently about around 4.0. So, most of the 1920s can be designated as one pitching era. However, as with 1920, Figure 3 shows that average ERA surged in 1929 and 1930. This makes those two years a poor fit with the rest of the data for their respective decades and brings up a problem with naming convention that needs to be addressed. A name without dates may not be understood by a reader. However, what is the proper name for a time that includes most of the years in a decade, (or decades) but not all? It is not accurate to call this period the 1920s (that would imply that 1920 and 1929 are included) and it is awkward to use just the years themselves without a name. With these considerations in mind, the convention of calling these eras “Most of” or “Mostly” their respective decades will be used. This is awkward, but more accurate than just using the actual decades. That said, it is interesting to note that the overall average ERA data conforms well to decade designations between 1920 and 1990.

The average ERA in 1929 and 1930 does not fit with the data starting in 1931. So as with 1920, these two years are designated as transition years. The overall average ERA between 1931 and 1941was slightly higher (4.18) than the Most of the Twenties Era. This result is in accordance with the generally held notion that the Mostly the Thirties Era was an offensive oriented era. Offense held sway until 1941 but average ERA declined considerably with the U.S. entry into WWII following the attack on Pearl Harbor in December 1941.

WWII and the Fifties

Figure 3 clearly shows that the average ERA for the war years, 1942 – 1946, is significantly lower (3.47) than the average ERA for the eras that precede or follow the war (4.18 and 3.98 respectively). From an Average ERA perspective, these five years must be recognized as a separate era. It is likely that the primary cause of the decline is the absence of over 500 professional ballplayers during the war. Bill James argues that the quality of baseball played during this period did not really live up to major league standards (9).

However, in 1947 average ERA increased to 3.89 and the average annual ERA stayed mostly between 3.75 and about 4.0 until 1962. This period (1947 – 1962) is designated Mostly the Fifties Era. The maximum ERA during this period was 4.36 in 1950 with the lowest annual average ERA of 3.7 occurring in 1952. The overall average ERA was just under 4.0 (exactly 3.98) for the era. Interestingly, this overall average ERA is very close to the overall average of 4.05 during the Most of the Twenties Era. This is somewhat surprising given the perception that the 1920s, powered by the heroics of Babe Ruth, are usually considered an offensive era. However, the data shows that the balance between pitching and hitting was roughly the same in the Mostly the Fifties Era as it was during the Most of the Twenties Era.

From the Sixties to the Nineties

The pitching situation changed dramatically in 1963 when the strike zone was expanded by the Rules Committee. Bill James argues in his eponymous New Historical Baseball Abstract that this was done to return the strike zone to its pre-1950 status but the effect was much more dramatic than intended (10). The expansion initiated a second deadball era. Average ERA fell to 3.39 in 1963 and stayed right around 3.5 for the next three years. That was not the end of the decrease in average ERA. Average ERA declined to 3.30 in 1967 before another significant decline in average ERA to 2.98 in 1968. Pitchers ruled the game to such an extent that 1968 is referred to as the Year of the Pitcher (19). In addition to their Cy Young awards, pitchers Bob Gibson and Denny McLain won both the MVP awards that year. Gibson’s ERA of 1.12 in 1968 was the lowest since the Deadball Era and the fourth lowest ever according to Baseball-Reference.com. This period of pitching dominance between 1963 and 1968 is designated the Low Sixties Era. 

The delicate balance between offense and pitching had swung too far toward hurlers. In response, the mound was lowered from 15 inches to 10 inches in 1969. This had the desired effect, as average ERA increased to 3.61 in 1969 and 3.89 in 1970. But after 1970 average ERA dropped dramatically again over the next two years to 3.26 in 1972 and then it increased again to 3.75 in 1973. The rapid changes in average ERA between 1969 and 1972 make this five-year period difficult to characterize so the period is going to be designated the Post Sixties Transition Period.

Nineteen seventy-three starts the longest pitching era in baseball history. After 1973, there is a twenty-year period where Average ERA stays mostly between 3.5 and 4.0. This era, which encompasses most of the 1970s, all the 1980s, and the first three years of the 1990s, has an overall average ERA of 3.82 for the entire era. Average ERA did spike to 4.29 in 1987 but immediately fell back down to 3.73 the next year. It is not unreasonable to think of this period as the ‘70s and ‘80s if you have looked at the data and know that that characterization is not completely correct. But in the interest of accuracy, this period will be designated Most of the Seventies, the Eighties, and into the Nineties Era.

The Nineties and Beyond

Between 1992 and 1994 average ERA increased from 3.75 to 4.46. This resulted in a 16-year period with the highest average ERA in baseball history. From 1994 to 2009 average ERA was 4.46. The natural question to ask is, why did ERA increase so dramatically? The immediate answer that comes to mind is the use of performance enhancing substances by players. However, Benjamin Chase argues that other factors also likely contributed to the increase in offense (5). These other factors include the expansions in 1993 (Colorado and Florida) and 1998 (Arizona and Tampa Bay), ballpark effects from new ballparks, better conditioned players, and umpires calling a smaller strike zone. It is difficult to untangle exactly what caused the offensive explosion during this period but the data is clear. The hitters had their best period in baseball history. Bill James’ designation of the Camden Yards Era is retained for this time because of all the new ballparks constructed during these years.

Starting in 2010, pitchers began to get the upper hand again. Average ERA decreased to 4.08 that year and then further decreased to 3.74 by 2014. But between 2014 and 2019 average ERA went back up to the Camden Yard Era territory with an average ERA of 4.51 in 2019. These rapid changes in ERA make the period from 2010 to 2019 difficult to characterize by the terms set out in this analysis. It will take a few more years of data to determine if the more historically average ERA of 2014 (3.74) or the higher average ERA of 2019 (4.51) predominates in the next few years.

Measuring the Variation in ERA During Different Eras

Although Table 1 includes the average ERA data for each era, the discussion here will focus on the standard deviation results. The first thing to note is that the standard deviations for the early eras are much higher than for the later eras. For example, the standard deviation of average ERA during the Spitball Era was the highest in the data set, 0.778. This reflects the big increase in average ERA that occurred when the pitching distance was increased to 60 feet 6 inches in 1893 and the subsequent decline to an average ERA of 2.37 in 1908. The lowest standard deviation of 0.093 occurred during the World War II Era. This is because the variation of average ERA was small during this short period. These two standard deviation numbers provide a range by which to judge the standard deviations of other eras.

To address the question posed earlier, (why are the data driven era designations an improvement?) the standard deviations during the Bill James era designations must be compared to the data driven era designations. The lowest standard deviation of average ERA (0.210) for the Bill James designations occurred during the Artificial Turf Era. With just two exceptions, the Deadball Era (standard deviation 0.284) and the Low Sixties Era (standard deviation 0.222), all of the standard deviations in the data driven designated eras are less than 0.210 which is the lowest standard deviation of the James designated eras. The standard deviations during the other data driven eras are 0.120, 0.200, 0.093, 0.170, 0.174, 0.140. These results are unambiguous. The data driven era designations are an improvement over the James era designations. This means that the average ERAs during the data driven designated eras are a better reflection of the overall ERA during the period than are the James designations of the average ERAs during his eras.

CONCLUSIONS

This paper examines and analyzes the average ERA in major-league baseball each season between 1871 and 2019. The data set 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 the entire time was 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 can also be 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. Other data driven pitching eras are shown in Figure 3 and listed in Table 1. The lowest overall average ERA of 2.82 occurred during the appropriately named Deadball Era (1904-1919). The increase in offense 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. 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 traditionally defined eras.

APPLICATIONS IN SPORT

Historical data by itself is meaningless. It needs to be examined and analyzed to understand the implications and applications of the numbers. This analysis shows that pitching data is better characterized by the eras proposed in this paper than conventionally designated eras. That provides anyone who is interested in baseball a better understanding of how pitching has evolved over baseball history.

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