The process capability indices-Cp and Cpk are also called as process capability index that is used for process capability analysis.
Process capability analysis is carried out to measure the ability of a process to meet the specifications.
A Histogram and Control Charts are the basic 7 QC Tools that are used in process capability analysis.
Six Sigma Methodology is used to improve process capability by process variation reduction.
Table of Contents
Process Capability Index: Cp and Cpk
Process capability – Cp : This is defined as the tolerance width divided by the total spread of process (6 Sigma).
Cp indicates the spread of variation present in a process.
Process Capability Index – Cpk: This is the capability index that accounts for the centering of the process and is defined as the minimum of Cpk upper and Cpk lower.
Cpk is a measure of process performance capability.
Cpk indicates shifting or closeness of process average from the target or mean value.
Interpretations of Cp and Cpk
When the Cp value is greater than 1 i.e. Cp value >1: The process spread is less and all products fall within the specification limit. Here the process is said to be quite capable of meeting the specification limit.
When the Cp value is equal to 1 i.e. Cp value =1: The process spread is little wide but running within the designed specification limit. Here the process is said to be just capable of meeting the specification limit.
When the Cp value is less than 1 i.e. Cp value <1: The process spread is large and most of the products fall outside the specification limit. Here the process is said to be incapable.
When Cp value equal to Cpk i.e. Cp=Cpk: The Process means is said to be at centre.
When the Cpk value is less than 1 i.e. Cpk value <1: Indicates that the mean of the process is shifted from target and defects will be produced.
When the Cpk value is greater than 1 i.e. Cpk value >1: The center or mean of the process may be shifted from target but still the process is capable of meeting design specification.
In order to achieve Six Sigma quality in the organization, we must reduce the variation in the process so as to achieve the desired value of Cp.
Note: Cp value should be ≥ 1.33 during the normal production process. And the recommended value of Cp is ≥ 1.67 during the new product development-NPD stage.
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