From: Stan Hilliard
Date: 14 Aug 2003
Time: 10:26:10
I find it useful to distinguish between two main applications that use sampling methodology in manufacturing: product disposition and process feedback/control.
PRODUCT DISPOSITIONS APPLICATIONS: Here the goal is to prevent delivery of an off-quality lot -- should it unfortunately be manufactured in the first place. In this case I think of the sampling plan and the information that it provides as a decision tool for disposition of existing product lots. This kind of plan honors the producer's p' (AQL) and the consumer's RQL.
PROCESS FEEDBACK/CONTROL APPLICATIONS: Here the goal is to assist employees to control of the process. Assuming that the hardware part of the process is capable when adjusted to target, and employee workmanship is capable of consistant good product when he/she is diligent, a sampling plan can provide feedback to employees as to when their diligence has slipped, to adjust hardware, or to correct current product. In this case, I think of the sampling plan and the information that it provides as part of the process itself. It helps to ATTAIN the quality levels of the process.
This kind of application does not honor the producer's p' (AQL fraction defective) because the employee can use the information to drive p' down.
Also, the RQL in such an application might be worse than the product goal. Not because of low standards, but that small lots, quick feedback, and economics may not allow the sample size required for the desired lot RQL.
To Elaborate: If you inspect a lot after it is too late to influence the quality of that lot, I advocate having a producer's point that depends on process capability, safety, and economics.
On the other hand, if a sampling plan is applied at the point where the processing responsibility of the employees is current -- say that the employee inspects his/her own work -- then it could be reasonable to design a sampling plan that neglects AQL in order to react to any observed imperfection.