CAPAs and Trending of Quality Data
Measurement Systems Analysis
Spec Setting, Tolerance Analysis and Robust Design
Statistical Process Control
Design of Experiments
- Dr. Wayne A. Taylor
- Ann Taylor
- 1 (847) 367-1032
- 1 (847) 367-1037
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Libertyville, IL 60048
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Statistical Procedures for the Medical Device Industry
STAT-12: Verification/Validation Sampling Plans for
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).
- Attribute Single and Double Sampling Plans for Proportion
- Variables Single and Double Sampling Plans for Proportion
- Selecting Sampling Plans for Proportion Nonconforming using
- Lower Confidence Limit for Percent Conforming—Attribute Data
- Lower Confidence Limit for Percent Conforming—Variables Data
- Sampling Plans for Proportion Nonconforming
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% 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
- 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
compare these sampling plans to other approaches including normal
tolerance intervals, capability analysis and confidence intervals on
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.