What is Process Capability Cpk, Cp, Ppk and Pp
One of the difficult topics to understand made simple, learn in simple words the basic concept of capability index. Also explained is the simplest method to calculate it and interpret the capability index value.
Capability is defined as the ability of a process to produce outputs that meet the product / service and / or the customer specifications. A capable process is a process whose spread on the bell-shaped curve is narrower than the tolerance range/customer specifications. Process capability studies are best conducted on processes that have reduced special and common cause variation to a minimum.
Why do we use process capability
Use of Process Capability
- Predict the extent of variability in a process.
- Choose from competing processes.
- Plan the interrelationship of sequential processes.
- Help establish periodic process control checks and adjustments.
- Test the theories of cause of defects during quality improvement programs.
- Certify suppliers.
The objective of process capability study is to establish a state of control over the process and then maintaining that state of control through time. When the natural process limits are compared with the specification range, any of the following action may result:
1. Do nothing : If the process limits fall well within the specification limits, no action may be required.
2. Center the process : When the process spread is approximately the same as specification spread or process spread is towards one side then adjustment to the centering of the process may bring the bulk of the product within specifications.
3. Reduce variation : It may be possible to check the variation source and work towards reducing.
4. Change the specification : Sometimes it is required to set tighter specifications and at times it may be required to relax the limits.
Capability measure for continuous data type
What is Cp: Calculation of Cp
Cp is the short run process capability and is determined using one or very few operators, same lot of material and single set of equipment.
- Cp compares product specification range with six-sigma range.
- Determines if process is capable of meeting specifications.
- Assumes process is centered between specification limits
Cpk is the short run process performance and is determined using one or very few operators, same lot of material and single set of equipment.
- Cpk compares product specifications relative to centre (X) of the process
- Similar to Cp in that it uses the standard deviation of the process, but does not need to have process centered to specification limits.
There are two types of Capability:
When a process capability is measured using one operator on one shift, with one piece of equipment and homogeneous supply of material, the process variation is relatively small. As we add more staff, more material, the factors impacting variation, overall variation increases in the process. So, the control limits based on a short term process are closer as compared to control limits based on the long term process.
Short term reflects “short term processing” and long term reflects what customer gets.
For attribute data we can not calculate Cp(k) and Pp(k) or sigma level directly but we can count good and bad products to calculate a %Defectives or PPM level. We can count defects and opportunities for a defect and calculate a DPMO level.
Defects per million opportunities (DPMO) of a process is the total no. of defects divided by total defect opportunities, multiplied by one million. Synonymous with PPM.
Defects : It is the failure of the process as per specification. It can be in a form of wrong information,, wrong opening of call , no closing …. Etc . Denoted by d
Unit (U) –The number of parts, sub-assemblies, assemblies, or systems inspected or tested.
Opportunity (O) –A characteristic you inspect or test. It is the maximum nos. of defects possible in one unit.
Defect (D) –Anything that results in customer dissatisfaction. Anything that results in a non-conformance.
Defects per opportunity – DPO: (D/(U * O))
Defects per million opportunity – DPMO : ( DPO * 1000,000)
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