Six Sigma is a statistical measure of variation in a process. We say a process has achieved Six Sigma if the quality is 3.4 DPMO (Defect per Million Opportunities). It's a problem-solving methodology that can be applied to a process to eliminate the root cause of defects and costs associated with it.

The main focus of Six Sigma is to reduce defects and variations in the processes. DMAIC and DMADV are the models used in most Six Sigma initiatives.


DMADV is the model for designing processes while DMAIC is used for improving the process.

The DMADV model includes the following five steps:
• Define: Determine the project goals and the requirements of customers (external and internal).
• Measure: Assess customer needs and specifications.
• Analyze: Examine process options to meet customer requirements.
• Design: Develop the process to meet the customer requirements.
• Verify: Check the design to ensure that it's meeting customer requirements

There are situations where we need to analyze what caused the failure or problem in a project. The fish bone or Ishikawa diagram is one important concept which can help you find the root cause of the problem. Fish bone was conceptualized by Ishikawa, so in honor of its inventor, this concept was named the Ishikawa diagram. Inputs to conduct a fish bone diagram come from discussion and brainstorming with people involved in the project. The following figure shows the structure of the Ishikawa diagram.

The most accurate method of quantifying variation is by using standard deviation. It indicates the degree of variation in a set of measurements or a process by measuring the average spread of data around the mean. It's more complicated than the deviation process discussed in the previous question, but it does give accurate information.