The historical leagues take various approaches to determining the best players in baseball history. They all use the Hardball program's statistical analysis engine, but in the case of the Decade and All-Time leagues they use seasonal notation to express an average of more than one year.
Presently, there are four types of leagues: All-Time, Generational, Decade and Annual. Other than the All-Time league, they all attempt to group players with contemporaries to assess relative worth.
There are a few common characteristics of each league.
A player's position is determined by his dominant position in the years used for the league. Thus, Joe Torre qualifies as a catcher in the All-Time league, though he played a few more games at third and first base as his career waned. And a player may qualify at one position in the All-Time league and another in a Decade league.
"Caught Stealing" data is not available for some years. A player does not get credit for any stolen bases in such a year. This may adversely affect players before 1940 such as Ty Cobb.
The All-Time and Generation leagues use the average of the players' 10 best seasons as seasonal notation. If a player lacked 10 full seasons, like Dizzy Dean, their career stats are still divided by 10 for seasonal notation. The league is updated each year to include and update active players.
The All-Time league does not include many players from before the turn of the century because the game was dramatically different and their stats, especially those of pitchers, would tend to skew the numbers for everyone. However, a few players of note, like Cy Young and Ed Delehanty, were included because of their historical interest.
Data for the Decade leagues is compiled by taking a player's total output for a given decade and dividing it by 10 for seasonal notation. This obviously works against players whose careers spanned more than one decade but didn't cover all 10 years of any one of them. Because of World War II, there is no league for the '40s.
Future Generation leagues are planned that will group players by the era they played in, but will include their 10 best seasons. Some players, like Pete Rose and Ty Cobb, may end up in more than one generation grouping.