820.250 Statistical techniques.

(a) Where appropriate, each manufacturer shall establish and maintain procedures for identifying valid statistical techniques required for establishing, controlling, and verifying the acceptability of process capability and product characteristics.

(b) Sampling plans, when used, shall be written and based on a valid statistical rationale. Each manufacturer shall establish and maintain procedures to ensure that sampling methods are adequate for their intended use and to ensure that when changes occur the sampling plans are reviewed. These activities shall be documented.

Interpretation:

When assuring that components and other products meet acceptance criteria, manufacturers may test either all components or may test a portion of the components using a sampling plan based upon an acceptable statistical rationale (820.250). A manufacturer shall be prepared to demonstrate the statistical rationale for any sampling plan used. Plans should be developed by qualified mathematicians or statisticians, or be taken from established standards such as ANSI Z1.4. It should be recognized that all sampling plans have a built-in risk of accepting a bad lot.

This sampling risk is typically determined in quantitative terms by deriving the "operating characteristic curve" for the selected plan. Each sampling plan has a characteristic curve. ANSI Z1.4 contains operating characteristic curves for sampling plans presented in the standards, and it can be used to determine the risk a sampling plan presents. A manufacturer should be aware of the risks the chosen plan presents. Operating characteristic curves are a means of graphically showing the relationship between the:

• quality of lots submitted for sampling inspection, usually expressed in percent defective, but may be expressed in defect per hundred units; and

• the probability that the sampling plan will yield a decision to accept the lot, described as the "probability of acceptance."

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Reference: www.FDA.gov

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