AX-R04 — Digit preference in declared purity

A chromatographic purity figure is the ratio of two integrated areas. It has no reason to land on a half or a whole percent, and across a large set of genuine measurements the terminal digit should be roughly uniform. It is not, on some certificates.

The rule requires a pattern rather than a single instance: a supplier must have at least three round-numbered declarations before any of them is flagged, because 99.0 % is a perfectly possible result once. Three times is a distribution.

Three explanations are consistent with the pattern. The figure may have been rounded for presentation from a real measurement, which is defensible but should be stated. It may be a specification being reported as a result — "≥99.0 %" becoming "99.0 %" somewhere between the laboratory and the certificate, which is the most common case in the index's experience. Or it may not correspond to a measurement at all.

The rule cannot distinguish between these. What it can do is put the pattern next to the independent measurements of the same lots, and let the reader see whether the round numbers are optimistic. Across the index they are: rounded declarations diverge from the independent median by a mean 0.63 pp against 0.29 pp for unrounded ones.