Q: Our quality team is trying to improve inspection efficiency and enhance supplier management by employing ISO 2859-3:2005 Sampling procedures for inspection by attributes — Part 3: Skip-lot sampling procedures.
Here are two questions on product qualification related to clause 5.2.1 Generic requirements for product qualification.
1. The standard requires that:
b) The product shall not have any critical classes of nonconforming items or nonconformities.
First, my understanding is that the risk level with any potential failure or nonconforming of the product should be low to customer — is this correct? Second, if a candidate product carries some critical features (dimension of mechanical product), but also carries a number of low risk features, can we apply the skip lot concept only to the non-critical features? And continue to perform lot-by-lot inspection with critical features? We are concerned the definition of “product” in the standard is a generic term and could be interpreted as feature of a physical product.
2. The standard requires that:
c) The specified AQL(s) shall be at least 0,025 %.
Does this mean the AQL value should be less than or greater than 0.025%? I assume “greater.” In our company, the most often used is AQL 1.0 and AQL 2.5, which I think meets the requirement.
We would greatly appreciate your help.
A: My name is Dean Neubauer and I am the U.S. Lead Delegate to Subcommittee 5 on Acceptance Sampling and co-author of Acceptance Sampling in Quality Control, 2nd edition., 2009. CRC Press. I hope I can help you.
Let’s start with question 1.
The general idea of skip-lot sampling is to reduce the number of times incoming lots inspected due to exceptional quality on behalf of the supplier. ISO 2859-3 states this in the beginning as:
The purpose of these procedures is to provide a way of reducing the inspection effort on products of high quality submitted by a supplier who has a satisfactory quality assurance system and effective quality controls.
The reduction in inspection effort is achieved by determining at random, with a specified probability, whether a lot presented for inspection will be accepted without inspection.
A skip-lot sampling plan is also known as a cumulative results plan. In general, such plans require certain assumptions to be met regarding the nature of the inspection process:
- The lot should be one of a continuing series of lots
- We expect these lots to be of the same quality
- The consumer should not expect that any lot is any worse than any of the immediately preceding lots
- The consumer must have confidence in the supplier not to pass a substandard lot even though other lots are of acceptable quality
Under these conditions, we can use the record of previous inspections as a means of reducing the amount of inspection on any given lot. ISO 2859-3 states the above in 5.1.1 Requirements for supplier qualification:
The requirements for supplier qualification are as follows.
a) The supplier shall have implemented and maintained a documented system for controlling product quality and design changes. It is assumed that the system includes inspection by the supplier of each lot produced and the recording of inspection results.
b) The supplier shall have instituted a system that is capable of detecting and correcting shifts in quality levels and monitoring process changes that may adversely affect quality. The supplier’s personnel responsible for the application of the system shall demonstrate a clear understanding of the applicable standards, systems and procedures to be followed.
c) The supplier shall not have experienced any change that might adversely affect quality.
The underlying assumption here is that the supplier quality is exceptional (low nonconforming level). The skip-lot plan is applied to each characteristic, or feature, separately. If several characteristics are present, then try to test for at least one of them. In your situation, if you have critical characteristics you should not be doing skip-lot inspection due to the risk of ignoring (skipping) a potentially dangerous lot. On the other hand, you can use a skip-lot plans for non-critical (major and minor) nonconformities and they will have different AQL levels associated with them. Your listing of AQLs of 1.0% and 2.5% are typical for major and minor nonconformities. Critical defects will typically have an AQL less than 1.0%, such as 0.25% to 0.65% (0% is preferred but theoretically unattainable as you would have to do a perfect 100% inspection, i.e., no inspection error).
The subclause referenced in question 2 states that the AQL level must be greater than or equal to 0.025%. Your levels of 1.0% and 2.5% can be used.
U.S. Lead delegate for Subcommittee 5 on Acceptance Sampling on ISO Technical Committee 69 on Applications of Statistical Methods.