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In determining compliance with a numerical limit, the calculated result of a test or assay is first rounded off 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, 0.15 indicates 2 significant figures and 140 indicates 3 significant figures). When rounding, only the digit immediately to the right of the last place in the limit is to be considered. If this digit is smaller than 5, it is eliminated and the preceding digit is not changed. If this digit is equal to or greater than 5, it is eliminated and the preceding digit is increased by 1. For example:

  • 0.145 rounded to 2 significant figures is 0.15
  • 0.284 rounded to 2 significant figures is 0.28
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