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previously reported proficiency test for the analysis of vitamin B in liquid dietary supplement demonstrated bimodality. The same trend has now been observed in four subsequent tests of this type. The trend would not so easily have been observed without applying a fit-for-purpose standard deviation that is more generous than that predicted by the Horwitz equation. Since originally reporting the bimodal problem and hypothesising its cause by incomplete enzymic digestion of riboflavin-5-phosphate, there has been a general improvement in the reporting of the higher mode. This is thought to correspond to free riboflavin following complete digestion of the sample. Several individual participants appear to have learned from the experience and have changed their reporting of the lower mode to the higher mode. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]