Addison Dick '22
Introduction
The 2017 Major League Baseball season marked 45 years since the American League adopted the designated hitter rule. At the MLB owners’ meetings last June, commissioner Rob Manfred said he believed that there was some movement on the issue from National League owners [1] . At the time of Manfred’s comments, NL teams in 2017 combined for more strikeouts than they did hits [1]. The purpose of this study is to analyze the difference in runs per game for AL and NL teams. Data from two sets of 46 MLB seasons, 1927 to 1972 as well as from 1973 (the first season with the DH rule) to 2018, was analyzed.
Hypothesis
American League teams have scored a significantly higher number of runs per game than National League teams since the adoption of the DH rule in 1973.
Statistical Methodology
To start, data was used from the “DH era” from 1973 to 2018 [2]. The data contains statistical averages for the AL and NL for every year since the DH rule was used. Averages were used because the number of teams in each league was not the same until 2013, so totals would not properly represent the difference in the leagues’ statistics.
The statistic analyzed was runs per game (or RPG). The table shows NL and AL averages over the course of the DH era as well as the mean difference between the two leagues.
| AL | NL | AL - NL | |
| Mean | 4.61913043 | 4.32652174 | 0.2926087 |
| Standard Deviation | 0.33763113 | 0.3011697 | 0.15268609 |
The following two graphs are visual representations of the difference in RPG in the DH era. The first graph shows the RPG for each league in every season since 1973. The second graph shows the difference between AL and NL RPG in each season. The difference is the NL RPG from the AL value.
The data shows that the American League is consistently scoring more RPG than the National League. The only year since the DH rule that the NL has scored more RPG was the 1974 season. The data, however, also shows that the difference between the leagues’ RPG is not as high as it once was. In the mid-1990s, the AL-NL difference was at its highest, including in 1971, when AL teams averages 0.71 more RPG than the NL. Since then, the difference has decreased, and in 2016 the AL-NL was at 0.08, its lowest point in 30 years.
Pre-DH Era Data
The next table shows the average values of RPG in the years leading up to the introduction of the DH [2]. The DH has existed for 46 years, so this data starts in 1927 to ensure that both tables includes the same number of seasons.
| AL | NL | AL - NL | |
| Mean | 4.50043478 | 4.3676087 | 0.13282609 |
| Standard Deviation | 0.54668681 | 0.39081217 | 0.39939767 |
The next two graphs are visual representations of the difference in RPG before the DH era. The first graph shows the RPG for each league in every season from 1927 to 1972. The second graph shows the difference between AL and NL RPG in those seasons. Once again, the difference is the NL RPG from the AL value (AL-NL).
This data shows a much different situation than the data from the DH era. There were many seasons in which the NL had a higher RPG than the AL. In fact, from 1953 to 1972, the AL only outscored the NL in six seasons. The difference in RPG in the DH era is notable. Under today’s rules, it would be shocking if the NL had a higher RPG than the AL.
Comparing the Means from the Pre-DH Era and DH Era
Pre-DH Era (1927-1972)
| AL | NL | AL - NL | |
| Mean | 4.50043478 | 4.3676087 | 0.13282609 |
| Standard Deviation | 0.54668681 | 0.39081217 | 0.39939767 |
DH Era (1973-2018)
| AL | NL | AL - NL | |
| Mean | 4.61913043 | 4.32652174 | 0.2926087 |
| Standard Deviation | 0.33763113 | 0.3011697 | 0.15268609 |
To compare the two eras, two-sample t tests were conducted. For all tests, an alpha value of 0.05 was used. The tests use one-tail P values because the purpose of the study is to observe if the AL has greater RPG value than the NL.
The first t test was between NL and AL RPG values in the pre-DH era. The one-tailed P-value equals 0.09415, which is greater than the alpha value. Therefore, the difference between the two leagues’ RPG values before the DH rule is not statistically significant.
The second test conducted was between the NL and AL RPG values in the DH era. The one-tailed P-value is less than 0.0001 and less than the alpha value, so the difference in the DH era is extremely statistically significant.
The final test was between the AL-NL difference in the two eras. The two-tailed P-value is 0.007, which is less than the alpha value. Thus, the difference is considered to be statistically significant.
Results
The difference between RPG for the AL and NL was not significant in the 46 years before the DH rule was adopted, yet it is quite significant in the DH era. Furthermore, the mean difference between the leagues’ values in the two eras is statistically significant. The hypothesis of this study is correct, as the statistical tests proved that American League teams have scored a significantly higher number of runs per game than National League teams since 1973.
Discussion
The data shows that the disparity between AL and NL RPG values increased in the DH era, but it is not obvious whether the DH rule is the sole agent of this change. While there may be other factors that contribute to the larger increase in the AL-NL difference, the DH rule has certainly affected the game of baseball around the world. Possible factors could be that the AL has better offensive players and the NL has better pitchers, but there is no apparent reason why one league would have higher offensive statistics other than the DH rule.
Commissioner Manfred wants to see more action and offense in baseball. This statistical analysis shows that adopting the DH rule in the NL would almost certainly create more scoring in the NL, reduce the significant difference between AL and NL offensive statistics, and allow all 30 teams in the MLB to play under the same rules in every game.
References
- https://www.chicagotribune.com/sports/baseball/ct-spt-mlb-cubs-dh-rule-sullivan-20180616-story.html
- Baseball Reference