Other materials that are coal(s) or coke(s) with documented
traceability to reference material (CRM) coal(s) or coke(s)
prepared by a national metrology body can also be used. Only
use material(s) with an assigned value and assigned uncertainty
for sulfur. The uncertainty expressed as the confidence interval
of the assigned value shall be less than the repeatability
specified in the appropriate section on Precision and Bias of
this test method.