Process and Measurement System Capability Analysis

This chapter explores process capability indices, and discusses another statistical quality control (SQC) technique called pre‐control. Pre‐control is used after a process is determined to be capable and operating in a state of statistical control. Pre‐control is used as a mechanism to reduce the amount of sampling and inspection required to validate that a process is producing products and services consistent with customer's expectations, as defined by specifications. A process capability index is a unitless measure that quantifies process capability: specifically, the relationship between the output and the designed tolerance or specifications. Using pre‐control also presents certain disadvantages when compared to the use of other SQC tools such as statistical process control (SPC). Measurement system analysis (MSA) is used to understand and quantify the variability associated with measurements and measurement systems. MSA is a more comprehensive analysis of quantifying variability components from gauge stability, gauge bias, gauge linearity, gauge repeatability, and reproducibility.