Capability Indices and Conformance to Specification: The Motivation for Using Cpm

ABSTRACT

Proposed is a perspective on quality that is different from but consistent with the current Six Sigma thinking. The focus is still squarely on the customer, but the concept of a “defect” is reevaluated. This new perspective can be used to help define key characteristics, process capability, and evaluate overall product quality using a loss function. This article focuses on the concept of process capability using this perspective. Central to the proposed quality perspective is ability to meet the customer's product specifications rather than ability to meet the component feature specifications. Current product and component quality metrics can drive the wrong decisions and wrong behaviors. This can add significant cost to internal processes and suppliers, and inadequately protect the customers from defective products. The new perspective defines component and process quality based on the probability of meeting the product specifications rather than the individual component feature specifications. Cpm is a simple process capability metric that can indicate the probability of meeting the customer's product specification. Cpm can be used to better estimate the cost of poor quality than can be estimated by using Cpk, and thus can be used to better manage product quality to the customer.

Keywords:

• Capability indices
• Cost of poor quality (COPQ)
• Cpk
• Cpm
• Critical to quality (CTQ)
• Defect
• Defective
• Design for Six Sigma (DFSS)
• Design specification
• Key characteristic (KC)
• Process capability
• Quadratic loss
• Root sum squares (RSS)
• Six Sigma
• Specification limits
• Statistical design methods (SDM)
• Taguchi
• Tolerances
• α-risk
• β-risk