Extension of the Limiting Quality Indexed Single Acceptance Sampling Plans for Attributes in High Precision Processes

This study investigated the extension of existing single acceptance sampling plans indexed by lot tolerance percent defective (LTPD) or limiting quality (LQ) with respect to their applicability to high precision processes. LTPD/LQ indexed sampling plans were extended to cover the very low fraction defective levels of high precision processes. Plans based on the ISO 2859-2 LQ indexed plans, Dodge-Romig LTPD indexed plans, and lot sensitive plans (LSP) were used as bases for the extensions. Target levels for LTPD/LQ were set at defective parts per million levels ranging from 20 ppm to 5000 ppm. The performance of the extended plans was evaluated using measures relating to level of protection afforded by the plans and efficiency in terms of amount of required inspection. The extended Dodge-Romig LTPD plans showed best performance among the three plans generated. Three selected plans from the extended Dodge-Romig LTPD sampling scheme were then subjected to simulation studies. Comparison of the theoretical and simulation data indicated that the plans behave more strictly than expected from theoretical calculations. The specific demands of high precision processes for appropriate sampling inspection plans that cover lower fraction defective levels, and at the same time satisfy the requirement for economy and efficiency of inspection were shown to be provided by this plan. These results contributed significantly to the manufacturing industry as it continuously strives to improve process yields and decreases fraction defective levels to respond to customer demands of better quality and improved performance