For example, Sandy Koufax turned in perhaps the most dominant five-year stretch of pitching in baseball history from 1962-66: 111-34, 33 shutouts, 1,444 strikeouts in 1,377 innings, ERA leader each season including three seasons under 2.00, and no-hitters four seasons in a row. But this performance was achieved in a pitcher-friendly home park during the Pitcher Era. It would be more difficult for him to achieve the same level of performance in the Home Run Derby Era, particularly in a hitters’ paradise home park like the new Citizens Bank Park in Philadelphia, nor would he be likely to complete games in the Home Run Derby Era at the rate that he did from 1962-66 (100 complete games in 176 starts). Conversely, it would be more difficult for Mark McGwire to replicate his HR performance from 1996-99 (52, 58, 70, 65) outside the Home Run Derby Era. In fact, most aspects of play are affected by the era in which a league is set.

The era in which a league is set determines the baseline rates for:

• Batting average
  
• Types of hits (proportion of singles, doubles, triples and home runs)
  
• Frequency of walks, strikeouts and hit batsmen
  
• Errors
  
• Pitcher endurance (particularly starting pitchers)

The era of play does not directly alter the likelihood that any particular tactic, such as a bunt or steal, will be employed, but rather, by altering the level and shape of offense generally, affects the decision-making process behind such tactics. So, for example, in an era that produces a lower-scoring environment, strategies that entail playing for a single run may tend to be used more frequently.

Standard Era (1920-1992)

The Dead Ball Era came to an abrupt end in 1919/20 as a result of the confluence of two factors: declining attendance and the fan appeal of the prodigious slugging of Babe Ruth.  There is some dispute about whether a “jackrabbit” actually was injected into the ball in 1919, when Ruth swatted a record 29 homers.  At least as important to the surge in offense were the practice of regularly introducing new balls into a game, which was adopted at this time, and the rule outlawing the “spitter” and other pitches involving defacement of the ball, which came into effect in 1920.  The major league batting average, which had remained around .250 from 1910-18, leaped to .268 in 1919 and over .280 in 1920, where it remained throughout the ensuing decade. The “Standard Era” is, in effect, a neutral playing environment, based on an average of statistical performance in the NL from 1920-1992. This is the era for Standard Leagues.

Pitcher Era (1963-1968)

The “Pitcher Era” encompasses the seasons 1963-68 for both leagues. Expansion always produces an upward “blip” in offense and the AL in 1961 was no exception, with the league ERA and HR per game increasing from 4.39 and 0.88 in 1960 to 4.53 and 0.95 in 1961. That upsurge, coupled with Roger Maris breaking Ruth’s single-season HR record, blinded owners to the fact that offense had, in fact, been in slow but steady decline for years. Instead they reacted by expanding the strike zone, ushering in the most pitching-dominated period in baseball since the “Deadball Era” (likely to be a future addition to Eras of Play).

The effect wasn’t immediately apparent. The drop in the numbers was modest in the AL in 1962 (ERA to 4.44 and HR/game to 0.96), with a comparable drop in the NL to 4.48 and 0.89 (from 4.52 and 0.97, despite the fact that 1962 was an expansion year for the NL, which should have resulted in an increase in offense). But in 1963 the numbers dropped precipitously – to 4.08 and 0.92 in the AL and 3.81 and 0.75 in the NL.

This era culminated in 1968 – the “Year of the Pitcher” – in which the AL and NL ERA and HR per game were a mind-boggling 3.41 and 0.68 and 3.43 and 0.55, respectively. Denny McLain won 31 games, Bob Gibson posted an ERA of just 1.12 with 13 shutouts, Don Drysdale threw 58.2 consecutive scoreless innings, and Carl Yastrzemski won the AL batting title with a record low mark of .301. The next season the strike zone was reduced to its former dimensions and the height of pitching mounds lowered, which (together with another round of expansion) restored a more reasonable balance between pitching and hitting.

What can you expect from a league in the Pitcher Era? Leagues will tend to have lower scores and league average ERA, lower batting averages and fewer home runs, among other impacts. In general, expect less offense and lower scoring games.

Home Run Derby Era (1993-2004)

The “Home Run Derby Era” is based on the National League seasons 1993-2004 (only the NL is used for this era and the Standard Era in order to avoid the differing effect on league statistics caused by the designated hitter in the AL.) The upsurge in home runs that began in 1993 was in full force by 1998, when Mark McGwire and Sammy Sosa both shattered the previous single-season home run mark, and culminated in Barry Bonds’s epic 73-HR season in 2001. In fact, half of all the 50-HR seasons in baseball history occurred between 1995 and 2002.

So how will the Home Run Derby Era differ from the Standard or Pitcher Eras? Obviously, there will likely be more home runs, and scores will be higher as offense rules the day. As one might expect, complete games will likely be fewer and earned run averages higher.