Six Sigma is a data driven problem solving approach with the objective of increasing profits by reducing the defects (improving effectiveness) in a process. Defect reduction happens in two ways - first, reducing the variation in the process, second, by moving the actual mean closer to the target mean.
Process Stability - refers to the consistency of the process to stay within the Control Limits. If the process distribution remains consistent over time, i.e. the outputs fall within the range (Process Width), then the process is said to be stable or in control. If the Outputs are spread across outside the limits, then the process is Unstable or Out of Control.
Nelson Rules is a set of rules to detect the presence of special causes or non-random behaviour in the process. These are commonly used to check for process stability in Statistical Process Control (SPC). They were first published by Lloyd S Nelson in 1984.
Applause for all the respondents - Suresh Kumar Gupta, Anuj Bhatnagar, Himanshu Sharma.