This text is from the second edition of ISO 9001, The Standard Companion. It is based on the now obsolete 1987 version of the ISO 9001 Standard. The third edition, based on the current 1994 standard, is available from Simply Quality.

4.20 Statistical Techniques

Use only Valid Statistical Process Control

If statistics are used to control product-affecting processes, the statistical techniques must be valid and procedures for their use must exist.


If you use procedures such as statistical process control (SPC) or statistical acceptance sampling, expect the auditor to ask about their validity. Note that this does not require the use of SPC or any statistical techniques.

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