# ISO 18391:2016 pdf free

**ISO 18391**:2016 pdf free.Geometrical product specifications (GPS) – Population specification

statistic defined from the characteristic values, obtained on the population of workpieces or the population of assemblies

Note 1 to entry: Population characteristics are used to consider the population of workpieces.

Note 2 to entry: Population characteristics are only statistically meaningful when the values are based upon global individual characteristics, see Example 2.

EXAMPLE 1 The arithmetic mean and the standard deviation of a global individual characteristic on the population of workpieces, are population characteristics.

EXAMPLE 2 The minimum circumscribed diameter has one unique value for a given cylindrical feature.Therefore a population characteristic based on this individual characteristic value will be statistically meaningful. The two-point diameter for a given cylindrical feature will vary within a range, dependent upon the form deviations of the feature. In this case, one population characteristic for two-point diameter cannot be defined from the population of values. However, it is possible to establish a population characteristic from the maximum two point diameter along the geometric feature, which is a global individual characteristic, together with a population characteristic from the minimum two-point diameter along the geometric feature which is another global individual characteristic.

An individual global characteristic is by default a univariate characteristic unless it defines a population characteristic. Surface texture characteristics, dimensional characteristics, and form characteristics by default define univariate population characteristics and orientation and location characteristics by default define multivariate population characteristics.

NOTE The range of values of a population characteristic considered univariate is larger than the range of the statistical combination of the components of the same characteristic considered multivariate, especially for the characteristics of orientation or location. Of course, if the population characteristic is acceptable without taking into account rule # 2a, it will also be acceptable when taking into account rule # 2a.

A population specification can be signed or not. Some population specification characteristics can be dependent on the condition given in the individual specification, but they can be also independent (see examples given in Table 3).ISO 18391 pdf free.