In New York the Yankees and Giants shared the Polo Grounds 1913-1922.
In St. Louis Cardinals and Browns shared Sportsman's Park 1921-1953. Then the Browns morphed into beautiful birds, the Baltimore Orioles.
In Philadelphia Athletics and Phillies (Blue Jays some seasons) shared Shibe Park 1939-1954. In 1955 the Athletics moved to Kansas City. Shibe Park was renamed Connie Mack Stadium 1953-1976 after the Athletics owner.
In Los Angeles Dodgers and Angels shared Dodger Stadium 1962-1965.
So how come the park factor is different for one team almost every season? Because baseball is played in non-uniform playing areas a park factor is needed to try to equate stats. There are actually two park factors, one for batters (BPF), one for pitchers (PPF).
BBF is equal in only 11 of the 65 shared seasons:
|1921||Polo Grounds IV||New York Yankees||New York Giants||102|
|1922||Polo Grounds IV||New York Yankees||New York Giants||102|
|1928||Sportsman's Park IV||St. Louis Browns||St. Louis Cardinals||103|
|1933||Sportsman's Park IV||St. Louis Browns||St. Louis Cardinals||106|
|1934||Sportsman's Park IV||St. Louis Browns||St. Louis Cardinals||107|
|1935||Sportsman's Park IV||St. Louis Browns||St. Louis Cardinals||104|
|1949||Shibe Park||Philadelphia Athletics||Philadelphia Phillies||97|
|1950||Shibe Park||Philadelphia Athletics||Philadelphia Phillies||98|
|1950||Sportsman's Park IV||St. Louis Browns||St. Louis Cardinals||104|
|1954||Connie Mack Stadium||Philadelphia Athletics||Philadelphia Phillies||100|
|1974||Shea Stadium||New York Yankees||New York Mets||99|
In St. Louis when the teams had the same BPF three consecutive years it was different each year: 106, 107, 104. Same thing in Philadelphia for two consecutive years: 97, 98. Oddly the Yanks with the DH and Mets are one of the 11 seasons when the teams had the same BPF.
|1921||Polo Grounds IV||New York Yankees||New York Giants||98|
|1922||Polo Grounds IV||New York Yankees||New York Giants||98|
|1938||Sportsman's Park IV||St. Louis Browns||St. Louis Cardinals||104|
|1939||Sportsman's Park IV||St. Louis Browns||St. Louis Cardinals||105|
|1940||Shibe Park||Philadelphia Athletics||Philadelphia Phillies||101|
|1941||Shibe Park||Philadelphia Athletics||Philadelphia Phillies||101|
|1941||Sportsman's Park IV||St. Louis Browns||St. Louis Cardinals||104|
|1946||Shibe Park||Philadelphia Athletics||Philadelphia Phillies||100|
Two instances when both BPF and PPF were the same:
|1921||Polo Grounds IV||New York Yankees||New York Giants||102||98|
|1922||Polo Grounds IV||New York Yankees||New York Giants||102||98|
Park factor is used to compute OPS+ and ERA+, two bedrocks of current conventional wisdom. I realize that parks change from one season to the next, even the configuration of the same park changes, and that there are different parks in the two leagues but this indicates how little we know about this important stat other than the vague notion that it somehow mysteriously takes the different and non-uniform playing areas into account in order to enable us to compare Albert Pujols and Lou Gehrig.
We really should have a much better understanding of park factor. Wins Above Replacement (WAR). All that stuff.