It is said that you don’t know how you are performing unless you measure the performance. It is also said that you don’t measure something unless you value it. If your process is valuable and adds value to organization, then there is always a need to measure its performance. Quantitative measurement of the process is the first step towards six sigma.
Sigma in Six Sigma term is nothing but standard deviation. Your process is said to be Six Sigma when center (mean) of your process is apart by six standard deviation from both lower and upper specification limits. Statistically, if you process is performing as mentioned above, then your process will not produce more than 3.4 defects per million opportunities. A defect in this case would be anything above or below the upper or lower specification limits given by the customer. Concept of Sigma and Z score is very powerful and helps us predict the performance of the process both in short and long term. Z is calculated by having mean subtracted from specification limit and then divide it by standard deviation. This will help predict rejections for corresponding area.
As you would have understood by now that Six Sigma focuses on statistical process control and reduce process defects by reducing variation within the process. The Six Sigma projects uses DMADV or DMAIC process for bringing improvement. DMADV is generally used when Six Sigma has to be implemented in new process unless existing process is poorly defined and not meeting customer objective at all whereas DMAIC improvement methodology is followed when project is done on existing process. It is needless to say that the implementation of done for both these strategies is done by Green Belts and Black Belts under mentorship of Black Belts.