Enhancements to a process capability analysis Six Sigma approach provide much organizational benefit not only for individual lean Six Sigma improvement projects but business performance metric reporting as well.
Traditional Process Capability Analysis Six Sigma Approach
A traditional process capability analysis six sigma methodology has issues:
- Not all statistical software programs provide the same answer for process capability calculations.
- Minitab calculations for process capability Cp and Cpk values depend upon the order individual values are entered.
- Equations in books are for normally distributed data . Computer programs can often address situations where data are not from a normal distribution; however, this is often not done in practice and can cause confusion when this form of reporting is conducted.
- Sample size and confidence intervals are not typically addressed.
Below is suggested an alternative to a traditional process capability approach that does not use Minitab’s or other statistical software process capability function.
One output available from a traditional statistical process capability analysis statistical program is estimated non-conformance rate relative to a specification. Since most people try to visualize the Parts Per Million (ppm rate) impact from any reported process capability index, reporting only a long-term ppm rate could be satisfactory, using transformations as needed. However, the 30,000-foot-level reporting method described below offers much more valuable.
Mathematically one of the big issues with a process capability analysis Six Sigma approach is how standard deviation is calculated. A value for standard deviation is a function of how data for the calculation are determined:
Additional information about traditional process capability analysis issues and resolution is described in:
- Link at the end of this webpage
Also, converting ppm rates to Sigma Quality Levels is not beneficial and can cause much confusion.
Enhanced Process Capability Analysis Six Sigma Approach
- With an assessment of process stability and process capability in one chart
- Using data from the most recent region of stability for process capability calculations; i.e., from the most recent process response
- With words that everyone can easily understand; i.e., estimated non-conformance rate when a specification exists
- Even when no specification exists
- Provides a futuristic statement for stable processes, where if future expectations are undesirable, then this metric improvement need pulls for a project tthat is to enhance a process
Options for easy creation of 30,000-foot-level reports: