Brief guide on Concept of Capability Indices - Cp, Cpk, Pp, and Ppk
What is Process Capability?
Process capability means the ability of the process to achieve desired results that meets the customer specification and statistical limits in a consistent manner. Process capability is defined based on the historical performance of the process. Process capability is one of the tools of Statistical Process Control (SPC).
In the fewest possible words — process capability indicates how efficiently a process can perform its intended purpose.
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Why is process capability important?
The business owner or process owner always wishes that the product that produces should give maximum benefit, consistency, less rejection, maximum yield, fewer stoppages, and less viability.
Monitoring process capability allows understood and evaluated the manufacturing process performance. Once the process is evaluated, it can be adjusted as needed to assure products meet the design or customer’s requirements. Capability Indices (Cp, Cpk, Pp, and Ppk) will help to understand the process behavior to reduce scrap, improve product quality and consistency and reduce the cost to production and the loss of cost because of poor quality.
Process capability was first introduced in Statistical Quality Control Handbook by the Western Electric Company (1956).
–“process capability” is defined as “the natural or undisturbed performance after extraneous influences are eliminated. This is determined by plotting data on a control chart.”
Following are widely used four indices to measure process capability and process performance:
Cp: Process Capability
Cpk: Process capability index
Pp: Process Performance
Ppk: Process performance index
Reference: ASTM E2281: Standard Practice for Process and Measurement Capability Indices
The statistics assume that the population of data values is normally distributed. Variability of the process can be observed as either short-term or long-term.
Cp and Cpk are measured based on short-term variability.
Pp and Ppk are measured based on long-term variability.
Process Capability/ Process Potential Index — Cp
Cp accounts only for spread or variation of the process
Process capability index or Centering Index — Cpk
Cpk accounts only for spread and location of the process
Tips for Cp and Cpk
Cpk will be always ≤ Cp
When process id perfectly centered, Cpk = Cp
Cp represents the highest possible value for Cpk
Cp requires 2 specification limits
Cpk can be calculated using single as well as both side specification limits
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The article originally published at tech-publish.com
