A variable-type skip-lot sampling plan for products with a unilateral specification limit

Acceptance sampling plans are practical quality management tools for lot sentencing. The rapid advancement of manufacturing technologies has reduced the defective rate substantially, and as a result, variables sampling plans with process capability indices (PCIs) have been adopted widely so that the required sample size can be decreased. Skip-lot sampling plan (SkSP), which evolved from the continuous sampling plan type-1, is applicable to bulk materials or products manufactured in successive batches or lots. It requires a smaller fraction of lots being examined, and the cost of inspection thus can be reduced. Hence, this paper consolidates the advantages of PCIs and SkSP, and proposes a skip-lot sampling plan of type-2, called SkSP-2, based on the one-sided PCIs for products with a unilateral specification limit. The proposed plan uses the exact sampling distribution of the estimators to develop a mathematical model for determining plan parameters, and the research prepares tables of plan parameters under different combinations of quality requirements and risk levels for quick reference in real applications. In addition, the results are compared with a conventional sampling plan to examine the contribution of the approach, and extensions to two types of SkSP are also presented. Finally, a graphical user interface is constructed to facilitate data input, criteria calculation and decision-making in practice.