Interpretation of Ph Eur rounding rule

[Terry]

Dear all,

As I compare the rounding rules specified in different pharmacopoeias, it was found that European Pharmacopoeia rule is quite confusing for me, here below are the words of interest:

In determining compliance with a numerical limit, the calculated result of a test or assay is first rounded to the number of significant figures stated, unless otherwise prescribed. The limits, regardless of whether the values are expressed as percentages or as absolute values, are considered significant to the last digit shown (for example 140 indicates 3 significant figures). The last figure of the result is increased by one when the part rejected is equal to or exceeds one half-unit, whereas it is not modified when the part rejected is less than a half-unit.

Here is one example, the limit for assay of a drug product is 100.0% - 102.0%, the raw result (not rounded yet) is 98.28568%. Shall I round the result to the same number of significant figures (4) as for the limit? The result would be 98.29%. If I round this result per USP rule, it would be 98.3%.

Thanks,

Terry

[tom jupille]

As far as I can tell, you are interpreting it correctly. And yes, it would have made more sense to specify decimal places rather than significant figures in this case.

[Terry]

Dear Tom,

Thanks very much for your clarification.

Regards,

Terry