Has anyone calculated or estimated it?
Is there a stat out there that hasn't been in baseball!? It probably isn't that hard, if you wanted, to find the batting average for left-handed hitters born on a Wednesday when facing a 2-2 count with a man on third. I digress though, yes, we can pretty easily look see the change in hitting, using the always amazing Baseball Reference which provides stats out the wazoo.
Here are some choice stats for the "Dead Ball Era" (roughly the first two decades of the 20th century) as well as some buffer on the sides to demonstrate the dip.
Year | BA | Runs | OPS | HR ---|---|----|----|----|----|---- 1900 | .279 | 5.21 | .705 | .22 1901 | .272 | 4.99 | .686 | .20 1902 | .267 | 4.44 | .665 | .16 1903 | .262 | 4.44 | .664 | .15 1904 | .247 | 3.72 | .622 | .13 1905 | .248 | 3.89 | .630 | .14 1906 | .247 | 3.61 | .621 | .11 1907 | .245 | 3.52 | .614 | .10 1908 | .239 | 3.38 | .602 | .11 1909 | .244 | 3.54 | .618 | .10 1910 | .249 | 3.83 | .644 | .14 1911 | .266 | 4.51 | .693 | .21 1912 | .269 | 4.53 | .695 | .18 1913 | .259 | 4.04 | .670 | .19 1914 | .254 | 3.86 | .659 | .19 1915 | .250 | 3.81 | .650 | .17 1916 | .248 | 3.56 | .638 | .15 1917 | .249 | 3.59 | .635 | .13 1918 | .254 | 3.63 | .642 | .12 1919 | .263 | 3.88 | .670 | .20 1920 | .276 | 4.36 | .707 | .26 1921 | .291 | 4.85 | .750 | .38 1922 | .288 | 4.87 | .749 | .43 1923 | .284 | 4.81 | .738 | .40
Just what the exact bounds of the "Deadball Era" are is something people like to debate, but as you can see, from the 1902 season through the 1919 season, the average never broke .270, while it hadn't dipped below that point previously since the 1892 season, which saw a jump from .245 to .280 the following year, in conjunction with several rules changes implemented for the 1893 season, including moving the mound from 50 feet to 60.5 feet, as well as exempting successful sacrifice plays from counting as at at bat, which helped stats as well. And from 1920 onwards, Batting Average wouldn't drop below .270 again until 1940. Likewise with runs, the dip there is even more telling, since while batting averages in the 1880s might have been lower than the 1890s, runs certainly weren't, with most seasons showing an average of over 5 per game, and likewise afterwards, while in the "modern" game, batting average rarely breaks .270 (three seasons in the past two decades), runs per game remains above 4 these days, and since 1919 has been a rarity (1976 was the last below, with 3.99, and I count nine total under four from 1920 onwards), a mark which, as you can see, was rarely broken during the "Dead Ball Era".
OPS is also quite telling, with OPS simply not breaking past .700, while previously having only a single season since 1893 (.681 in 1898), and not seeing it again until 1942 when a streak of sub .700 OPS lasted through the 1946 season. As with runs, recent OPS hasn't dipped that low in awhile, with 1989 clocking in at .695. If you look at post-1892 OPS, the "Dead Ball Era" has nine of the ten lowest OPS (1968 ranks at #9 with its dismal .639). Which brings us to Home Runs, which is possibly the biggest "Wow that ball was dead" stat. I mean, just look at that sudden upshoot from 1920 onwards! After two decades of rarely breaking .20, "wham", balls getting jacked up there left and right, and unlike many of these other statistical measures, we have never seen a single season go anywhere near that level again. No season since 1920 has hit .20, and since the 1920s only a single season (1943, at .37) has even been below .40. Currently, the year-to-year average flits around 1.00 (in no small part to the change in park sizes as noted by /u/AGATECH). While homeruns were not too common in the game before the "Dead Ball Era" either, looking only from 1900 onwards, 1900 to 1919 rank as #1-20 for lowest rate of homeruns (1875 and 1876 tie at 0.06 for all time lowest if you care to know).
So anyways, that should give you some idea of the deadness of the "Dead Ball Era". Like I said, you can debate when it started a bit, since, as you see, the stats don't show a really clear date to point too (and if you check out the stats into the 1890s, there are also some low points previously), but it is pretty crystal clear that 1919-1920 saw its end, with a marked shift in hitting - led in no small part by Babe Ruth who led both years in Runs, RBIs, Homers, and OBP/SLG/OPS. So while in some cases stats would again hit "Deadball Era" numbers (the war years saw some of the most notable downturns with many players off in uniform), it simply hasn't been with such frequency, and perhaps most importantly, never in the realm of the big bomb into the stands.
All stats from Baseball Reference
Rules information from Baseball Almanac
Great info here (notice the size of the parks):
http://billsportsmaps.com/?category_name=baseball-1908-al-nl-dead-ball-era
Looking at the leaguewide statistics tables at http://www.baseball-reference.com/leagues/MLB/bat.shtml may be of interest.
The dead ball era is conventionally reckoned as lasting from 1901-1920, although it's quite justifiable to draw less sharp boundaries - runs per game started trending downward in 1898, and although the traditional end boundary is perfect with regard to home runs, not until 1930 was 1900's run per game total equaled. (and 1901 and 1920 are associated with other major baseball changes which make it useful for periodization - notably the founding of the American League for the former, and Landis becoming commissioner for the latter.)