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Statistics that have been adjusted for all-time have all of the adjustments for a single season, plus two more.
One adjustment normalizes the average fielding numbers over time. Historically, the fielding share of total defense has been diminishing with time - more walks, more strikeouts, and more home runs means less work for fielders. In the single-season adjustments, fielders from before WWII have a lot more value than fielders today; the all-time adjustments have attempted to remove that temporal trend.
The second adjustment is for league difficulty. League quality has generally increased with time. Each league has been rated for difficulty and compared to a trend line defined by the post-integration National League.
In addition to the adjustments for season, an adjustment is made for league difficulty.
Adjusted for all-time appears in the following BP Glossary Categories: