### Introduction

It has been an interesting 2022 baseball season for many reasons including the deadened baseball discussed in the previous post. Jayson Stark and Eno Sarris in a recent The Athletic article note that the rates of each of the three true outcomes (home runs, strikeouts, and walks) are all down in the 2022 season and try to explain why. One contributing factor to this phenomena could be the change in the designated hitter (DH) rule in 2022 — for the first 162-game season, all teams (both National and American leagues) use the designated hitter for all games.

Thinking about the designated hitter rule, I thought it would be interesting to explore the use and benefit of the DH rule over the history of its use in Major League Baseball. Here are some questions that will guide our exploration:

- What fraction of plate appearances have been taken by designated hitters and has this fraction changed over the seasons of use?
- Where does the designated hitter hit in the batting order and has this changed over the DH period?
- What is the benefit of a DH hitter compared to other non-pitchers, and has this benefit changed over the 49-season use of the DH?

### A Short History of the DH

For those who are not familiar with the designated hitter, here’s a brief history of its use in Major League Baseball. For many seasons, the pitcher would be part of the batting lineup, typically batting 9th in the order. Starting with the 1973 season, the American League teams were allowed to substitute a player in the batting lineup for the pitcher. This “designated hitter” would only bat — that is, this player would not play in the field. From 1973 through 1996, designated hitters were used in the American League and not in the National League. When interleague play started in 1997, the designated hitter was used for interleague games in the American League team home park. This DH practice for interleague games continued for seasons 1997 through 2021, although in 2020 the MLB allowed the use of DH for all games as a health and safety measure during the COVID-19 pandemic. Starting with the 2022 season, the MLB commissioner announced that a “Universal DH” (DH in both leagues) would be used, and I assume that the Universal DH will be used in future MLB seasons.

### Use of the DH Over 49 Seasons

In the early seasons, the designated hitter was only used in the American League. Since the DH is taking one of 9 batting positions and the DH is used in only about half of the games (only the American League), I would estimate that 1 / 18 = 0.055 of the PAs would include the DH for these early seasons. When the DH is introduced in interleague play, there would be an increase in the use of DH in the away interleague games for National League teams, but this would be counterbalanced with a decrease in DH use for the away interleague games of American League teams. In 2020, the DH was used for all teams, so I’d expect the DH to be used in 1/9 = 0.11 of the PAs.

With this background, here is a plot of the fraction of DHs in plate appearances for all of the DH seasons excluding 2020. I add a vertical line that shows the season where DHs first appeared in interleague games. Frankly, I don’t understand the pattern in this graph. The fraction of PAs with DHs is about 0.056 from 1973-1976, increasing to 0.06 between 1977-1992, dropping to 0.053 from 1998-2012, and back to 0.056 in recent seasons. The sizes of these changes are small, but it seems that more exploration is needed to understand this pattern. Why, for example, would the fraction of DH use drop after being introduced in interleague games in 1997?

Since the DH is replacing the pitcher in the batting lineup, a related graph is the proportion of all PAs that are filled by pitchers. The pattern of this graph is interesting, especially the increase in pitchers batting from 1990 to 2005. But the proportion of PAs with pitchers batting has steadily decreased in the last 10 seasons — in the current season the proportion of PAs with pitcher batting is close to 0.

### Position of DH in Batting Order

Generally, I think the DH is an above-average hitter who is placed relatively high in the batting lineup. Is that true? For all DH plate appearances each season, I compute the fraction that bat in each position of the batting order. Below I graph the fraction of DH appearances in batting position 1-3, the fraction of appearances in positions 4-6, and the fraction of appearances in positions 7-9 as a function of the season. The general patterns are shown by smoothing curves. It is most likely for the DH to bat in positions 4-6, next in positions 1-3, and then in positions 7-9. It does appear that the DH batting position is changing over seasons. From 1973 through 2021, the fraction of DHs batting in positions 1-3 is increasing and the fraction of DHs batting in positions 4-6 is decreasing.

### Comparing the Batting Performance of DH with Non-Pitchers

How well do the designated hitters perform? Let’s compare the average performance of DHs with the average performance of non-DHs (not pitchers) over the 49-season period using various measures.

We look at walks, home runs, strikeouts and wOBA — we define the DH effect as

DH effect = Rate of DH – Rate of non-pitchers (not DH)

This graph displays the DH effect for each of the four measures (BB, HR, SO, wOBA) for the 49 DH seasons together with smoothing curves. Note that the points tend to fall above the red line of a zero effect — the takeaway is that designated hitters are more likely to get a walk, hit a home run and get a strikeout, and their wOBA values are higher. It is interesting that the walk and wOBA DH effects appear to peak about the 1995 season. The HR DH effects seem pretty constant over the 49 seasons. Also the wOBA DH effects are close to zero for recent seasons.

### Data Notes

- I am using Retrosheet play-by-play files for the DH seasons for this exploration. For a given season, I focus on batting plays (variable BAT_EVENT_FL is TRUE). The key variable is BAT_FLD_CD that indicates the fielding position of the batter. A value of BAT_FLD_CD = 10 indicates a designated hitter and BAT_FLD_CD = 1 indicates a pitcher is batting.
- The Retrosheet variable BAT_LINEUP_ID indicates the position (1 through 9) of each hitter in the batting order.
- The Retrosheet variable EVENT_CD tells us the outcome of the plate appearance. Using this variable, I can pick out strikeouts, walks, home runs, and hit values that are connected with wOBA weights.

### Comments

- Since the designated hitter appears to be a rule that will stay in MLB, it is worthwhile to take a statistical look at its use and how designated hitters tend to be better hitters than other non-pitchers.
- One could explore
**DH effects at an individual level.**For example, it would be interesting to look for best and worst DH hitters. Are there particular players that hit much better as a DH than as they do as a position player? - I am currently puzzled with the
**strange pattern of the DH rate over seasons**, but probably can be explained if I look at the DH rates over leagues and teams. - The
**roles of pitchers and non-pitchers**are currently changing in the 2022 season. It is rare for a pitcher to bat, but it is increasing common for non-pitchers to pitch innings in blowout games. (As I am writing this, Toronto is defeating Boston 28-5 in an extreme blowout game.). Also we currently have a great two-way player Shohei Ohtani who excels in both pitching and hitting.

*I think* the weird changes over time are due to different numbers of teams in the leagues. So 76 had 12:american & 12:national, whereas 77 had 14:american, 12:national

Thanks — that makes perfect sense.