Quality Q&A : What are sampling plans and how are they employed for inspections?
By: Tim Kieselhorst, Operations Manager, Pro QC International
What are sampling plans and how are they employed for inspections?
The US Department of Defense created Military Standard 105 to control the quality of parts they purchased. In the mid 1990’s, they decided to let non-governmental organizations take over this effort. The most commonly used standard today is ANSI Z1.4-2003. Most businesses and third-party quality service providers like Pro QC use sampling plans based on these statistical tables. More specifically, Pro QC uses the ANSI Z1.4-2003 standard.
Because checking 100% of the quantity would take considerable time and money, Pro QC employs random sampling throughout the production run during inspections. But, how many samples should be selected? If checking too few, the inspector may not find quality problems. Therefore, the objective is to be reasonably certain of good quality while being efficient in time and cost.
The ANSI sampling plan has three primary inspection levels, which include reduced, normal and tightened. These are referred to as General Levels I, II and III. General Level II is used for approximately 90% of inspections and is designed to balance quality, time and cost in the most efficient manner. The ANSI sampling plan also contains four Special Levels, which are used for tests that may be destructive or take an extended amount of time to complete. For example, Special Levels are often chosen for packaging integrity testing or functional evaluation. An inspection often requires the use of both General Level II and Special Level 3, for example. Which level is chosen depends on a variety of factors, such as the confidence or experience you have with the supplier.
AQL (Acceptable Quality Limit) is the maximum percent of defects allowed in each shipment. Determining this often requires knowing what is acceptable in the marketplace. For example, aerospace and automotive industries require much higher levels of quality, which are generally expressed in PPM (parts per million). Your Pro QC account manager can work with you regarding which levels would work best for each component of the inspection. Detailed specifications are key in this regard and provide field engineers (and suppliers) with documentation that includes requirements regarding the product, its function and packaging.
Defects noted during inspections generally fall within three categories, which include critical, major and minor. Critical defects have potential to cause harm to the user, major defects would be considered unacceptable to consumers and minor defects are not to specification but might not affect a consumer’s purchasing decision.
Different AQLs can be set for each category of defect and would be detailed within the specifications. As an example, critical defects fall at 0.10 AQL and are not allowed for most consumer products. And, the AQL for major defects and minor defects may be set at 2.5 and 4.0 respectively. Using the tables located within the sampling plan provides exact details as to whether or not a shipment passes or fails the inspection.
For additional information, contact your account manager. The ANSI Z1.4 2003 sampling tables are also available for your reference on the Pro QC website, within Quality Resources.