“If you can’t describe what you are doing as a process, you don’t know what you’re doing”
W.Edwards Deming
Process capability describe us how much good products are produced in a process and centering of the process around its mean.
A statistical measure of the inherent process variability for a given characteristic.Process capability refers to the ability of a process to produce a defect-free product or service. Various indicators are used-some address overall performance, some address potential performance.
Assessing capability involves evaluating process variability relative to preset product or service specifications. Before calculating process capability we must ensure that our processes are stable and statistically controlled through SPC chart.
Process Capability is a tool to determine how much standard deviations are fit between mean and nearest control specification limit.it is an extended calculation for Z-score.In statistical field Z score is an essential distribution applicable for normal distribution with sample size greater than 30. CpK can be calculated through Z score
CpK=Z-Score/3
When describing a process, it is important to identify sources of variation as well as process segments that do not meet performance standards. Capability analysis is a useful tool because it illustrates the centering and spread of your data in relation to the performance standards and provides a statistical summary of process performance. Capability analysis will help you describe the problem and evaluate the proposed solution in statistical terms.
CpK – the number of standard deviations between the mean and the nearest specification limit
Once a process is under control and shows very little variation, process capability studies can be run to calculate the maximum capability of the process. Once a process is running near its maximum capability, making any additional changes to the process is usually not economical.
CP—This is one of the first capability indexes used. The “natural tolerance” of the process is computed as 6σ. The index simply makes a direct comparison of the process natural tolerance to the engineering requirements. Assuming the process distribution is normal and the process average is exactly centered between the engineering requirements,
A CP index of 1 would give a “capable process.” However, to allow a bit of room for process drift, the generally accepted minimum value for CP is 1.33. In general, the larger CP is, the better. For a Six Sigma process, that is, a process that produces 3.4 defects-per million- opportunities including a 1.5 sigma shift, the value of CP would be 2
Capability analysis is used with continuous data whenever you need to compare actual process performance to the performance standards. You can use this tool in the Measure phase to describe process performance in statistical terms. In the Improve phase, you can use capability analysis when you optimize and confirm your proposed solution. In the Control phase, capability analysis will help you compare the actual improvement of your process to the performance standards.
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