Most of us undergo some form of evaluation in the workplace, whether it’s an informal check-in or annual performance review. These assessments are designed to monitor our performance and determine areas for improvement. However, as great as a stellar annual performance review may make us feel, it certainly doesn’t mean we’ll never make mistakes on the job or won’t need to improve.
Processes themselves can be reviewed through capability and performance assessments. As we dive deeper into the world of processes, keep in mind that a capable process does not guarantee defect-free performance.
The Difference Between Process Performance and Capability
The process capability index can be expresses as a sigma level, where a lower sigma level indicates lower capability, and a higher sigma level indicates greater capability. The goal of the Six Sigma methodology is to reach the highest level of PERFORMANCE possible. Statistical measurements are also useful for measuring process capability and performance and can be used to determine if a process is ABLE (i.e., capable) to meet customer requirements. Cp, Cpk , as with the process Sigma level, are indicators of process capability index while Pp, and Ppk are part of process performance index – how defects did the process produce or probability that he would have produced defects. The key difference centers around what a process is “capable” of doing versus what it “did”.
Process capability assesses the process’s capability when in statistical control (the ideal case), i.e., when there are no special causes causing it to depart from what it is capable of doing. The Sigma-level of a process does the same but looks at the case that process may shifted somewhat – specifically that the means may shift by 1.5 standard deviations.
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What about process performance? Use Ppk to assess performance whether there is or isn’t any signs of special cause events affecting the process. Here you want to know what it did, not what it is capable of doing so that a condition of calculating Ppk isn’t that the process is statistically stable unlike the Cp and Cpk calculations. It is what it is!
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Where Control Charts Come into Play
While a process might fare well on the capability index as of Cpk = 2 would indicate, that doesn’t mean that it’s time to wash your hands of the process and forget about defects. Sure, a Cpk = 2 is a highly capable process – a “Six Sigma Process”, but it’s not immune to producing defects. While the capability index tells us is that the process has the capability to meet requirements, but not that it is will meet them again and again. A process still needs attention and monitoring for control, even after it’s capable. So, how do you identify the causes of defects?
If you guessed the statistical process control chart, you’d be correct. These charts, created by Walter Shewart in the 1920s, can be used to identify disturbances or special cause variation to the process that may be causing defects. The premise is simple: you record data to pinpoint an anomaly with process performance and root out special cause variation.
With a control chart, you can use historical data to create the upper control limit line, lower control limit line, and center line, which marks the average. As you plot the data in order, you’ll have a visual history of process changes and any inconsistencies or indications of disturbances to the process. Remember that consistent process variation indicates common cause variation while a presence of variation beyond that indicates special causes of variation are present. Some level of common cause variation is unavoidable. However, the best processors even learn how to reduce it. The secret sauce for learning to do that begins by first addressing the special cause variation and deepening one’s knowledge of cause and effect.
You can use a control chart for more than determining process stability. You can also pinpoint and address errors as they arise to prevent negative outcomes and defects as well as to guide your quality improvement efforts. So, while a process capability index Cpk of 2 is a great starting point, to achieve a truly world process, requires diligence in managing the processes with the use of regular statistical assessments of their stability and actions to understand and address them.
If you enjoyed this article, be sure to sign up for NWCPE’s upcoming Statistical Process course, Module SS 503.