Different approaches to setting the standard deviation for proficiency assessment

There are, as described in ISO 13528 , essentially five approaches to setting the standard deviation for proficiency assessment , i.e. the denominator of the z-score which controls the acceptable range of participant results:

• Prescribed value: set at a value required for a specific task of data interpretation, or it may be derived from a requirement given in legislation.

   

• By perception: set at a value that corresponds to the level of performance that the PT provider, and associated scheme advisors, would wish laboratories to be able to achieve.

   

• From a general model: derived from a general model for the reproducibility of the measurement method.

   

• From the results of a precision experiment: calculated from the repeatability and reproducibility of a standardised method used in the PT.

   

• From the participants' data obtained in the PT scheme: derived from the results reported in the PT round or in a number of pooled rounds of the same PT scheme.