A statistic was invented by Henry Chadwick in 1867 that pre-dates slugging percentage but figured the average of total bases. This was the basis for the modern slugging percentage. The modern slugging percentage that we use today uses that total bases per at-bat rather than per game as Chadwick’s statistic used in the 1800s. The modern slugging percentage was adopted by the National League in 1923 and by the American League in 1946.