Understanding how Cp Cpk are used to （了解如何使用Cp肌酸磷酸激酶来）.pdf

Understanding how Cp Cpk are used to ensure quality The purpose of this application note is to explain what Cp and Cpk are and how these statistical tools can be used to prevent shipping marginal products. Most people have at least heard of Cp or Cpk, but few people really understand the concepts behind the calculations. This is valuable information not just for Quality Managers, but also for the Engineers who design new products, as well as Purchasing Agents who have to maintain a list of qualified vendors. In fact the 1994 revision of ISO 9000 requires suppliers to evaluate their subcontractors on the basis of their ability to meet subcontract requirements (Section 4.6.2). Under the method of quality acceptance that is currently used by most organizations (including the U.S. government) the customer determines the requirements for an item and sets up specified limits within which an item must fall to be acceptable. Anything falling within the specified limits is considered good; anything falling outside the limit is bad. This is the go/no-go approach that Genichi Taguchi calls the “goalpost”mentality. This system is illustrated in Figure 1: Part A is in the center of the specification range, part B is just inside the specification limit, part C is just outside the specification limit, and part D does not even come close to the specified range. In this system parts A and B are both considered equally good parts, Parts C and D are both considered equally bad parts. With this approach there is no measure of goodness and product of marginal quality is routinely accepted. Figure 1 Organizations must stop using the “goalpost”approach and instead use a procurement process that defines quality requirements in terms of the distribution about the target value. Taguchi has stated this idea in terms of the loss function. The loss function essentially