Process Capability Studies

Process capability studies tells you how well your process or product meets specifications. Process capability baseline numbers predicts current and future defects for any given measurement characteristic.

The term for process capability is CpK.

CpK Guidelines

Have at least 30 measurements
Assure the data is normally distributed
Use sample standard deviation
Be sure to calculate on both upper and lower specification

CpK Calculation elements

Easy to calculate process capability if you have these 3 items:

mean
standard deviation
specification limits

CpK Equation

Lower number of the 2 below equations

(High Specification � Data mean) / (3*standard deviation)

or

(Data mean � Lower Specification) / (3*standard deviation)

CpK Interpretation

The closer the data average is to the nominal specification, the higher the Cpk.
The smaller the standard deviation, the higher the CpK
The closer the average is to the upper or lower specification the lower the CpK

Process capability and defects.

Definition of a defect: When a part is beyond the specification, the part is consider a defect.

See the below table for calculation of defects per million parts after calculating the CpK.

Process capability studies are a basic tool of process improvement.

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Robert Broughton
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