Abstract
Primary methods of measurement play an important role in metrology because they provide the essential first link in the chain of traceability from the abstract definition of a unit of the International System of Units (SI) to its practical use in measurement. A definition of a primary method of measurement has been developed by the Consultative Committee for Amount of Substance (CCQM) that distinguishes between those methods that measure a quantity directly and those that measure the ratio of two quantities. Methods that do not fulfil the definition of being primary also play an important role in measuring amount of substance. In some cases, it is appropriate to refer to these as secondary methods.