Time-reversal Test

The time-reversal test checks whether an index-number method gives internally consistent results no matter which period is chosen as the base. Concretely, if the index for year 1 is computed using year 0 as the base, and then the index for year 0 is computed using year 1 as the base, multiplying the two results together should give exactly 1 — each comparison should exactly "undo" the other: