.
When studying a process, are SPC charts necessary to demonstrate that a process is stable?
[Annex A – section A.2]
“A capability study is used to determine whether a process is stable and capable. It involves collecting samples over a period of time. The average and standard deviation of each time period is estimated and these estimates plotted in the form of a control chart. These control charts are used to determine if the process is stable.”
[Annex A - section A.3]
“Control charts are also used as part of a capability study to demonstrate that the process is stable or consistent.”
[page 151]
“[…] the control chart is a test of the hypothesis that the process is in a state of statistical control.”
[page 199]
“Clearly, if control limits for current or future production are to be meaningful, they must be based on data from a process that is in control.”
[page 328]
“When […] there is no direct observation of the process or time history of production, then […] we can only estimate the distribution or the product quality characteristic or the process yield; we can say nothing about the dynamic behavior of the process or its state of statistical control.”
[page 349]
“Histograms, probability plots, and process capability ratios summarize the performance of the process. They do not necessarily display the potential capability of the process because they do not address the issue to statistical control, or show systematic patterns in process output that, if eliminated, would reduce the variability in the quality characteristic. Control charts are very effective in this regard. The control chart should be regarded as the primary technique of process capability analysis.”
[page 351]
“Sometimes the process capability analysis indicates an out-of-control process. It is unsafe to estimate process capability in such cases. The process must be stable in order to produce a reliable estimate of process capability. When the process is out of control in the early stages of process capability analysis, the first objective is finding and eliminating the assignable causes in order to bring the process into an in-control state.”
[page 247]
“Control charts must be used to verify the stability of the process. Estimates of process mean and standard deviation are meaningless unless stability is verified. Also, using graphical techniques such as a histogram to verify the normality of the data (or the lack thereof) does not give any indication of the stability of the process.”
[page 197]
“In establishing control of a process, the effort is usually initiated with a ‘no standard given’ control chart to investigate the process. This, then, allows development of data on the process, which may be used to eliminate assignable causes, establish control, and to estimate process parameters. After 30 to 50 successive points have remained in control, it is possible to establish (essentially) known, stable values of the process parameters."