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       Statistical Procedures for the Medical Device Industry    Statistical Procedures for the Medical Device Industry   

STAT-12: Verification/Validation Sampling Plans for Proportion Nonconforming

Purpose

This procedure provides tables and instructions for selecting sampling plans for FDA process validation and design verification to ensure they are based on a valid statistical rational.  These determine the samples size and acceptance criteria.  They make confidence statements like 95% confidence the process or device is more than 99% reliable or conforming. These sampling plans are often referred to as confidence reliability sampling plans.  They are for the statistical property proportion conforming or nonconforming.  They require that requirements be established for individual units of product. They apply to design verification (STAT-04), process validation (STAT-03), validation of a pass/fail inspections (STAT-08) and CAPA effectiveness checks (STAT-07).

Appendices

  1. Attribute Single and Double Sampling Plans for Proportion Nonconforming
  2. Variables Single and Double Sampling Plans for Proportion Nonconforming
  3. Selecting Sampling Plans for Proportion Nonconforming using Software
  4. Lower Confidence Limit for Percent Conforming—Attribute Data
  5. Lower Confidence Limit for Percent Conforming—Variables Data
  6. Sampling Plans for Proportion Nonconforming

Highlights

     Attribute Sampling Plans

  • Appendix F of STAT-12 contains tables like the one shown below for 95%/99%  - 95% confidence of more than 99% reliable or conforming.  This is equivalent to 95% confidence of less than 1% nonconforming.  This table contains attribute single and double sampling plans. 

                    95/99 Attribute Sampling Plans

  • 95% confidence of more than 99% conformance means there is a 95% chance of rejecting a 99% conforming product/process.  99% conforming is therefore an unacceptable level of quality designed to fail.

  • All the above sampling plans, if they pass, allow the same confidence statement to be made.  They offer the same protection against a bad product/process passing.  They are all equivalent from the customer/regulatory point of view.  They differ with respect to their sample sizes and their chances of passing a good product/process.  The decision of which confidence statement to use should be based on risk and must be justified.  The choice of which sampling plan to use for a given confidence statement is a business decision.

  • The AQLs in the above table are nonconformance levels that have a 95% chance of passing the sampling plan.  They are useful in deciding which sampling plan to use.  Historical data can be used to estimate the nonconformance rate and then matched to the AQL.  If historical data is not available, data from similar products or processes can be used.  If there is no good estimate of the nonconformance rate, stay away from the top of the table.  These are the hardest plans for a good product/process to pass.   

  • The top plan, n=299, a=0 offers the lowest sample size.  It minimizes the sample size.  However, it also has the lowest AQL.  It maximizes the chance a good product/process will fail.

  • The double sampling plans have a first sample sizes not much greater than the top single sampling plan.  They offer a good compromise between sample size and the chance of false rejection of a good product/process.  They are generally preferred to the single sampling plans.

  • Attribute sampling plans are always applicable.  For measureable characteristics, they make no assumption about the underlying distribution of the data.  However, they have higher sample sizes.  When the measurements follow the normal distribution or can be transformed to the normal distribution as described in STAT-18, variables sampling plans can be used to dramatically lower the sample size. 

 Appendix F also contains tables of variables sampling plans.  They will be covered in a separate article..  Future articles will also compare these sampling plans to other approaches including normal tolerance intervals, capability analysis and confidence intervals on capability indices.

These tables and all the procedures can be licensed individually or as a group by a company so that they can use them for or as part of there company procedures.  This requires paying a 1-time license fee as described at Company Licenses.  If your company is using these tables, please make sure they have been properly licensed.  Below is a previous version of the table.

            


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Last modified: September 08, 2017