During the years 1913 to 1954, and again in 1962 to 1964 and 1974 to 1975, at least one American League and National League team played in the same stadium. This allows for a comparison of the relative hitter/pitcher friendliness of the other parks in each league. Using the park run factors in Total Baseball edition 1, I compared the run factor for the field or fields used by both leagues, then calculated the average run factor for the remaining stadia.
Overall, the National League yards were more hitter friendly than the AL's during the years 1913-1954. There was not much difference found in the later years studied.
From 1913 to 1922, both the Giants and the Yankees played their home games in the Polo Grounds. For the first 7 of these seasons, the average run factor for the other 7 NL parks was 101, while in the AL it was 100. In 1920, the Cardinals moved from Robison Field, which slightly favored pitchers, to Sportsman's Park, one of the best hitter's parks around. This increased the difference in the two leagues to 3 (101 NL vs. 98 AL.)
In 1923, the Yankees opened Yankee Stadium, which was pitcher friendly, leaving the hitter haven Polo Grounds. During the years 1923-1937, the average run factors for the non-St. Louis fields were 99.6 in the NL and 98.4 in the AL.
Beginning in 1938, the Phillies shifted their home games to Shibe Park, sharing it with the Athletics until the latter team moved to Kansas City in 1954. During those years, the 6 NL parks not used by AL teams had an average run factor of 99.8 while the remaining AL yards scored a 98.1. It is interesting that the American League tended to have higher scoring games during the years 1915-1943 despite the fact that its ball parks were harder to score in than the NL's.
Following the Browns' move to Baltimore and the A's hightailing it to KC, there were 16 separate ballparks in use in the two leagues. Not until the expansion of the leagues in the early 1960's did two teams again share a stadium. This was in Dodger Stadium/Chavez Ravine, which the Angels occupied during the years 1962 until 1964. With 10 teams in each league, the impact of one park on the overall average is lessened. There was a difference of 0.4 in those 3 years, with the NL parks again more hitter friendly.
The final example of two teams using the same stadium occurred in 1974 and 1975, while Yankee Stadium was being renovated. The Yankees shared Shea Stadium with the Mets for those two years. Despite the perception that the AL had better hitters' parks, with several older fields such as Fenway and Tiger Stadium still in use, the average park factors for the nonshared yards were an identical 101 in each league.
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