What are the common methods of sampling inspection?

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How to do sampling inspection? What are the common methods of sampling inspection?

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Sampling inspection is a method for determining whether a lot or population should be rejected or accepted based on the number of defective parts found in a sample of the lot. If the number of defective components exceeds a predetermined threshold, the lot is rejected.

Common methods of sampling inspection based on different classifications:

Classification by inspection scheme
– Lot-by-lot sampling inspection: a predetermined number of samples are taken for inspection from a lot of products manufactured with the same materials and machinery. The result of the sample quality inspection determines whether the lot’s quality is acceptable or not. Typically, this type of lot-by-lot inspection is known as sampling inspection. In order to manage the conditions of the manufacturing process, a small number of products from various stages of production are sampled for inspection. This inspection, also known as a process inspection or spot check, examines the conditions of a process. It is conducted for specific management purposes, including the creation of a management diagram for quality control and the collection of data for cycle time analysis. Controlled sampling inspections, as opposed to other sampling inspections, examine machines, systems, operators, and operating procedures. This facilitates the early detection and resolution of any operational, equipment, or system-related issues. As a result, a controlled sampling inspection requires typically fewer samples than other sampling inspections.

Classification by acceptance level determination
– Sampling inspection based on operating characteristics: an inspection plan that defines producer protection and buyer guarantees, as well as meets the requirements of both producers and buyers. This sampling scheme safeguards the producer by assigning a small value to the percentage of high-quality lots that are incorrectly rejected (producer’s risk). On the other hand, this sampling scheme protects buyers by establishing a small value for the small percentage of incorrectly accepted poor-quality lots (consumer risk). As seen on an OC curve, the producer’s risk is generally set at 5%, while the consumer’s risk is set at 10%.
– Sampling inspection with adjustment: adjusts the quality guarantee to purchasers by decreasing or increasing the sampling inspection based on historical inspection quality records. The sampling inspection with adjustment can be indexed by AQL or LQ or conducted according to a skip-lot inspection plan. ISO 2859-1, ISO 2859-2, and ISO 2859-3 outline the sampling method for inspection with adjustment.
– Rectifying inspection: a 100 percent inspection is conducted on a lot rejected in a sampling inspection. Therefore, this scheme is inapplicable to products that cannot undergo 100 percent inspections.
– Sampling inspection for continuous production: an inspection scheme that sequentially inspects products that are produced continuously rather than in batches. The plan for sampling inspection may begin with a 100 percent inspection and then switch to sampling inspection after a predetermined number of conforming products. If a defect is found, the mode of inspection reverts to a comprehensive examination.

Classification according to sampling technique
– The acceptability of a lot is determined based on the results of a single sampling inspection.
– Double sampling inspection: determines the acceptability of a lot based on the results of a second sampling inspection when it is unclear based on the results of the initial inspection whether the lot is accepted or rejected.
Multiple sampling inspection: uses more samples than double sampling inspection. Each time, the sample size is determined and compared to the specified standard. This mode of inspection determines acceptance or rejection based on the specified inspection count and classifies results as acceptable, unacceptable, or uncertain.
– In multiple sampling inspection schemes, sequential sampling inspection determines acceptability with greater specificity. Individual sequential inspection, where a single item is sampled, and grouped sequential inspection, where a fixed number of items are sampled simultaneously, are the two methods for sequential sampling inspection.

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