Reducing variation

All around us is variety. A driver has variation when parking his car; train arrival times have variation; the human race exhibits variation; and products that come out of a process are never the same. Every process exhibits variation.

“Uncontrolled variation
is the enemy of quality.”

When we talk about process capability, we distinguish between two concepts: “Process Capability” and “Process Performance. Process capability shows what the process can do, while process performance shows what the process actually does. The difference between these two results indicates the potential for improvement.

Before a Process Capability study can be conducted, it is important to ensure that the process is stable and that the data are normally distributed. A process capability study therefore consists of three parts:

  • Stability:
    the first test is to determine whether there is special variation that affects the process. Is the process stable? This is performed using a “Run chart.
  • Normality:
    the second key is to determine if the process is generating normal data. Are the process data normally distributed? This is carried out with a normality test.
  • Capability:
    then it is possible to examine whether the process has enough capability. Is the process capable of meeting customer or business specifications? This is done with a process capability study.

The performance of a process can be indicated by a sigma number that indicates the percentage of error-free products. A process performing at the 6 sigma level shows that 99.99966% of products are within specification and only 0.00034% are out of specification (defective).

This is equivalent to 3.4 defects per million opportunities to make a mistake (defect). Nevertheless, it is not necessary for all processes to perform at this level. The Six Sigma philosophy is to achieve breakthroughs in quality and performance. A process that initially performs at the 2 sigma level (equivalent to 31% defects or 308,538 DPMO) and after a Six Sigma project performs at the 3 sigma level (equivalent to 6.7% defects or 66,807 DPMO) is a successful Six Sigma project because the project resulted in significant improvement.

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