Process Analysis - Process (Machine) Capability Analysis - Introductory Overview
Quality Control describes numerous methods for monitoring the quality of a production process.
However, once a process is under control the question arises, "to what extent does the long-term performance of the process comply with engineering requirements or managerial goals?" For example, to return to our piston ring example, how many of the piston rings that we are using fall within the design specification limits? In more general terms, the question is, "how capable is our process (or supplier) in terms of producing items within the specification limits?" Most of the procedures and indices described here were only recently introduced to the US by Ford Motor Company (Kane, 1986). They allow us to summarize the process capability in terms of meaningful percentages and indices.
The computation and interpretation of process capability indices will first be discussed for the normal distribution case.
If the distribution of the quality characteristic of interest does not follow the normal distribution, modified capability indices can be computed based on the percentiles of a fitted non-normal distribution. With the Process Analysis module, you can fit various specific non-normal distributions (such as Weibull, log-normal, Beta, Gamma, etc.) as well as general non-normal distributions by moments.
Order of business
It makes little sense to examine the process capability if the process is not in control. If the means of successively taken samples fluctuate widely, or are clearly off the target specification, then those quality problems should be addressed first.
Therefore, the first step toward a high-quality process is to bring the process under control, using the charting techniques available in Quality Control.