This article describes how to calculate Process Sigma and is an essential initial step in every Six Sigma project.
Steps to Calculate Process Sigma
In the traditional Six Sigma methodology, Define and Measure (from DMAIC) are the first two stages of the project. This article describes how the activities that go into these two stages result in the calculation of process sigma at their conclusion.
Definition of Defects and Opportunities
A defect is the lowest level of fault that is visible to a customer. An opportunity is the number of ways in which a defect can take place. Let us consider an example of a telecom company. The lowest defect a customer can recognize is say, one minute of downtime on the phone. The number of ways in which this downtime can occur is as follows:
1) 365 Days in a year
2) 24 hours per day
3) 60 minutes per hour
Therefore the Number of Opportunities = 365 * 24 * 60 = 525,600
Calculating the Process Yield
The process yield is calculated as the percentage of the total available opportunities in which the process has been executed successfully (defect-free). The company has taken a survey of its customers and found the average downtime to be about 1,000 minutes in a year. The process yield is calculated as follows:
Process Yield = (Total Number of Opportunities - Total Number of Defects) / (Total Number of Opportunities) * 100
i.e. Process Yield = (NOpp – NDef) / NOpp * 100
Therefore, here is for our example:
Process Yield = (525,600 – 1,000) / 525,600 * 100 = 99.80
Calculate Process Sigma From Process Yield
The next step is to obtain the process sigma from the process yield by using a sigma conversion table. The process sigma corresponding to 99.80 is about 4.4 Sigma. This article explains the significance of the sigma level of any process and an understanding of the "6" in six sigma. A conversion table for process yield to process sigma is given below for your reference.
Interpretation of Sigma Levels
Now that you have evaluated the defects, opportunities, process yield and corresponding sigma level, the next stage is utilizing the sigma level figures.
A process sigma level has two applications:
- An indicator of current quality levels
- Scope available for future improvements (difference between current sigma level and 6 sigma)
As process improvement measures are implemented, the number of defects will reduce and, consequently the process sigma level will increase. These steps are carried out as part of the improve phase of the DMAIC methodology.