Many detailed statistics are based on play-by-play accounts accumulated by Retrosheet. These totals may be
incomplete
(3.6% of all plays from 1912 to 1968 are missing or have
incomplete accounts, click link for year and team summary of data
completeness) for some players prior to 1969 and even complete seasons may not match the
official totals due to errors in both the official totals and the play-by-play accounts.
About the SABR Home Run Encyclopedia