Page "Major League Baseball Rookie of the Year Award" Paragraph 3

From 1947 through 1956, each BBWAA voter used discretion as to who qualified as a rookie.

In 1957, the term was first defined as someone with fewer than 75 at bats or 45 innings pitched in any previous Major League season.

This guideline was later amended to 90 at bats, 45 innings pitched, or 45 days on a Major League roster before September 1 of the previous year.

The current standard of 130 at bats, 50 innings pitched or 45 days on the active roster of a Major League club ( excluding time in military service or on the disabled list ) before September 1 was adopted in 1971.