Variation is everywhere around us. A driver has variation when parking his car; the arrival times of trains contains variation; the human race exhibits variation and products that come from a process are never the same. Every process exhibits variation.

The less the variation of a process, the better we can predict the outcome and the more predictably we can make a statement about the quality of the process. This is why Six Sigma has a strong focus on reducing variation.

“Uncontrolled variation is the enemy of quality”


In process capability, we make a distinction between two concepts: ‘Process Capability’ and ‘Process Performance’. Process capability shows what the process can perform, while process performance shows what the process actually performs. The difference between these two results indicates the possibility of improvement.

Before a study can be carried out into process capability (Process Capability study), 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 a special variation that influences the process. Is the process stable? This is carried out with a ‘Run chart’.
  • Normality:
    the second test is to determine whether the process generates normal data. Are the process data normally distributed? This is carried out with a normality test.
  • Capability:
    Consequently is it possible to examine whether the process has sufficient capability. Is the process able to meet customer or company specifications? This is carried out with a process capability study.

The performance of a process can be indicated by a sigma number that indicates the percentage of flawless products. A process that performs at the level of 6 sigma shows that 99.99966% of the products fall within the set specifications and that only 0.00034% fall outside the specification (defective).

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