Acceptance sampling
Encyclopedia
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. It is usually done as products leave the factory, or in some cases even within the factory. Most often a producer supplies a consumer a number of items and decision to accept or reject the lot is made by determining the number of defective items in a sample from the lot. The lot is accepted if the number of defects falls below where the acceptance number or otherwise the lot is rejected.
A wide variety of acceptance sampling plans are available.
, were developed by Harold F. Dodge
and others and became frequently used as standards.
More recently, quality assurance
broadened the scope beyond final inspection to include all aspects of manufacturing. Broader quality management system
s include methodologies such as statistical process control
, HACCP, six sigma
, and ISO 9000
. Some use of acceptance sampling still remains.
Plans have known risks: an acceptable quality limit
(AQL) 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).
; a random sample of size 
is selected from the lot; and an acceptance number 
is determined. If it is found the number of nonconforming is less than or equal to 
, the lot is accepted; and if the number of nonconforming is greater than 
, the lot is not accepted. The design of a single sampling plan requires the selection of the sample size 
and the acceptance number 
.
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.
for the same indexed AQL.
Quality control
Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...
technique used in industry and particularly the military for contracts and procurement. It is usually done as products leave the factory, or in some cases even within the factory. Most often a producer supplies a consumer a number of items and decision to accept or reject the lot is made by determining the number of defective items in a sample from the lot. The lot is accepted if the number of defects falls below where the acceptance number or otherwise the lot is rejected.
A wide variety of acceptance sampling plans are available.
History
Acceptance sampling procedures became common during WWII. Sampling plans, such as MIL-STD-105MIL-STD-105
MIL-STD-105 was a United States defense standard that provided procedures and tables for sampling by attributes based on Walter A. Shewhart, Harry Romig, and Harold Dodge sampling inspection theories and mathematical formulas...
, were developed by Harold F. Dodge
Harold F. Dodge
Harold French Dodge was one of the principal architects of the science of statistical quality control...
and others and became frequently used as standards.
More recently, quality assurance
Quality Assurance
Quality assurance, or QA for short, is the systematic monitoring and evaluation of the various aspects of a project, service or facility to maximize the probability that minimum standards of quality are being attained by the production process...
broadened the scope beyond final inspection to include all aspects of manufacturing. Broader quality management system
Quality management system
A quality management system can be expressed as the organizational structure, procedures, processes and resources needed to implement quality management.-Elements of a Quality Management System:# Organizational structure# Responsibilities# Methods...
s include methodologies such as statistical process control
Statistical process control
Statistical process control is the application of statistical methods to the monitoring and control of a process to ensure that it operates at its full potential to produce conforming product. Under SPC, a process behaves predictably to produce as much conforming product as possible with the least...
, HACCP, six sigma
Six Sigma
Six Sigma is a business management strategy originally developed by Motorola, USA in 1986. , it is widely used in many sectors of industry.Six Sigma seeks to improve the quality of process outputs by identifying and removing the causes of defects and minimizing variability in manufacturing and...
, and ISO 9000
ISO 9000
The ISO 9000 family of standards relates to quality management systems and is designed to help organizations ensure they meet the needs of customers and other stakeholders . The standards are published by ISO, the International Organization for Standardization, and available through National...
. Some use of acceptance sampling still remains.
Rationale
Sampling provides one rational means of verification that a production lot conforms with the requirements of technical specifications. 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 limit
Acceptable quality limit
The acceptable quality limit is the worst tolerable process average in percentage or ratio, that is still considered acceptable: that is, it is at an acceptable quality level...
(AQL) 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).
Acceptance sampling for attributes
A single sampling plan for attributes is a statistical method by which the lot is accepted or rejected on the basis of one sample. Suppose that we have a lot of size; a random sample of size is selected from the lot; and an acceptance number is determined. If it is found the number of nonconforming is less than or equal to , the lot is accepted; and if the number of nonconforming is greater than , the lot is not accepted. The design of a single sampling plan requires the selection of the sample size and the acceptance number .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
Sample size
Sample size determination is the act of choosing the number of observations to include in a statistical sample. The sample size is an important feature of any empirical study in which the goal is to make inferences about a population from a sample...
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. Compared with attribute sampling plans, these often use a smaller sample sizeSample size
Sample size determination is the act of choosing the number of observations to include in a statistical sample. The sample size is an important feature of any empirical study in which the goal is to make inferences about a population from a sample...
for the same indexed AQL.
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
- Sampling Procedures and Tables for Inspection by Variables for Percent Nonconforming, ANSI/ASQ Z1.9-2008
- Sampling Procedures and Tables for Inspection by Attributes, ANSI/ASQ Z1.4-2008
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 Evdence 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