Acceptance sampling
Acceptance sampling uses statistical sampling to determine whether to accept or reject a production lot of material. It has been a common quality control technique used in industry and particularly the military for contracts and procurement.
A wide variety of acceptance sampling plans are avilable.
History
Acceptance sampling procedures became common during WWII. Sampling plans, such as Mil-Std-105, were developed by Harold F. Dodge and others and became frequently used as standards.
More recently, quality assurance broaded the scope beyond final inspection to include all aspects of manufacturing. Broader quality management systems include methodologies such as statistical process control, HACCP, six sigma, and ISO 9000. Some use of acceptance sampling still remains.
Rationale
Sampling provides one rational means of verification that a production lot conforms with the requirements of a technical specification. 100% inspection does not guarantee 100% compliance and is too time consuming and costly. Rather than evaluating all items, a specified sample is taken, inspected or tested, and a decision is made about accepting or rejecting the entire production lot.
Plans have known risks: an acceptable quality level and a rejectable quality level (LTDP) are part of the operating characteristic curve of the sampling plan. These are primarily statistical risks and do not necessarily imply that defective product is intentionally being made or accepted. Plans can have a known average outgoing quality limit (AOQL).
=Attribute plans
MIL-STD-105 was a United States defense standard that provided procedures and tables for sampling by attributes (pass or fail characteristic). MIL-STD-105E was cancelled in 1995 but is available in related documents such as ANSI/ASQ Z1.4, "Sampling Procedures and Tables for Inspection by Attributes". Several levels of inspection are provided and can be indexed to several AQLs. The sample size is specified and the basis for acceptance or rejection (number of defects) is provided.
Variables plans
When a measured characteristic produces a number, other sampling plans such as those based on MIL-STD-414 are often used. Comparred with attriute sampling plans, these often use a smaller sample size for the same indexed AQL.
References
Books
- Pyzdek, T, "Quality Engineering Handbook", 2003, ISBN 0824746147
- Godfrey, A. B., "Juran's Quality Handbook", 1999, ISBN 007034003
- Squeglia , N L, Zero Acceptance Number Sampling Plans, Fifth Edition, ASQ Press, ISBN 978-0-87389-739-6
ASQ Standards
27. Sampling Procedures and Tables for Inspection by Variables for Percent Nonconforming ANSI/ASQ Z1.9-2008
28. Sampling Procedures and Tables for Inspection by Attributes ANSI/ASQ Z1.4-2008 Sampling Procedures and Tables for Inspection by Variables for Percent Nonconforming ANSI/ASQC Z1.9-1993
ASTM Standards
- ASTM E105 Standard Practice for Probability Sampling Of Materials
- ASTM E122 Standard Practice for Calculating Sample Size to Estimate, With a Specified Tolerable Error, the Average for Characteristic of a Lot or Process
- ASTM E141 Standard Practice for Acceptance of Evidence Based on the Results of Probability Sampling
- ASTM E1402 Standard Terminology Relating to Sampling
- ASTM E1994 Standard Practice for Use of Process Oriented AOQL and LTPD Sampling Plans
- ASTM E2234 Standard Practice for Sampling a Stream of Product by Attributes Indexedby AQL