ISO 18391:2016 defines rules to establish and to indicate population specifications, which are used to specify conditions on population characteristics, which are established from a set of characteristic values obtained one on each workpiece of a population of workpieces.
A population specification (as applied to a population of workpieces considered as a collection and not as individual items) can be seen as a complementary requirement to the individual specification (as applied to each workpiece considered as individual items). Population specifications express the statistical hypotheses used on the population of workpieces.

NOTE 1 A population specification is a complement to an individual GPS specification.

NOTE 2 ISO 18391:2016 is not intended to mandate a given tolerancing method or how to calculate tolerance values. Its intent is to specify tools to allow the expression of population specifications.