Question

7. When studying a process control chart tracking one variable, what is meant by the statement, "The process is in a state of statistical control"

Answer 1

The statement that a process is in a state of statistical control means that assignable or special causes of variation have been removed; characteristic parameters like the mean, standard deviation, and probability distribution are constant; and process behavior is predictable. If the Mean, mode and median values are same then the process is with in statistical control.

It may be special cause variation, where one or a few causes predominate to shift either the location or the dispersion of your process. The presence of special causes (detected using a control, or process behaviour, chart) tells you that your process is out of control or unpredictable.

A Process Is in a State of Statistical Control when:
No special causes are affecting the process ( all values are between +/- 3 sigma with some other rules).You have to start by removing special causes, so that you have a stable process to work with. But then comes the real fun, and often the most substantial benefits: it is time to improve the process, so that even common cause variation is reduced.