OPTIMIZING FREQUENCY AND NUMBER OF CONTROLS FOR AUTOMATIC MULTICHANNEL ANALYZERS

Sampling strategy fundamentally influences the effectiveness of qualit y control with control charts. This study shows a simple approach for optimizing the control strategy for automatic multichannel analyzers t hat takes into account cost-efficiency considerations. Our main focus is on the frequency of controls necessary. The methods used are based on a field study (on a Hitachi/BM 747), the views of experts, and comp uter simulations of customary cost-models together with a survey of th e literature. We found that industrial cost-models are applicable only with distinct limitations, but-unlike the test-yield model-they offer consistent solutions. On the basis of the field study and the opinion s of experts, we adjusted the control strategy to account for inadequa cies in the theoretical models. The combined result is that, for effec tive operation, the number of samples between controls may reach value s up to 100 and should not require controls more often than every 30 s amples on comparable multichannel analyzers. For adequate statistical performance, a simple 3-SD Shewhart chart usually requires not more th an two controls of the same material at each time.