The process capability calculation would over-estimate the defect rate if the data were used in this form. Note the difference between the observed histogram and the curve representing a normal distribution curve. Note that the subsets referred to in the figure are not the small groups of transactions occurring in close increments of time used to estimate short-term process capability.ДЇigure 3: Summary for Orders Without Valid Letters of Credit The flowchart in Figure 1 shows the logic of calculating the process capability starting with a set of continuous data. It is less common in a transactional environment when tracking financial information or cycle time. The normal, or Gaussian, distribution is commonly observed in data involving a physical measurement of some kind such as length of machined bars, weight of widgets or the average number of manufacturing defects per week.

The known characteristics of the distribution.

The process capability calculation is based on: Expressing a business problem as a defect rate typically provides a more direct way to communicate with stakeholders and members of the project team. The defect rate, expressed as DPMO (defects per million opportunities), is part of the common language of Six Sigma. The goal of a process capability calculation is to use a sample of items or transactions from a business process to make a projection of the number of defects expected from the process in the long term.

Turn the continuous data into discrete data.

Mathematically transform the data and specification limits.

Divide the data into subsets according to business subprocesses.

Although effective analysis of data that is not distributed normally is possible, completing one of the action steps below is beneficial for some projects to create a more useful data set:

During a Six Sigma project, the defect rate of the business process is calculated twice, first during the Define phase to show the impact of the problem and again during the Control phase to show how the process has improved.

How to calculate process capability