Capability Analysis: When is it used?

Use the standard method of capability analysis when you can answer "yes" to all these questions:

Is the system stable? An unstable system is constantly changing; therefore, information obtained from it will be unreliable over time. With a control chart, always check for stability before analyzing capability.
Are the individual values of the variables normally distributed? Capability analysis for non-normal distributions is handled by computers or by using a capability analysis sheet. The capability analysis sheet method is described as a variation at the end of this section. Test for normality with a histogram.
Do you want to know how the system performs in relation to the specification limits? Control charts (with the exception of the X-MR) do not give a direct comparison with specification limits, but such comparisons are essential to understanding overall system performance, to predicting future performance, and to monitoring system improvements.

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