BS ISO 7870-8:2017 pdf free.Control charts Part 8: Charting techniques for short runs and small mixed batches.
The business aim of statistical process control (SPC) is to control and improve quality, increase productivity and reduce cost. The principal graphical tool of SPC is the control chart. There arc three main classes of control charts: Shewhart, cumulative sum cusum) and exponentially weighted moving average (F.WMA).
NOTE Cusum control charts are dealt with in ISO 7870.4 and EWMA in ISO 7870.6.
The Shewhart control chart provides a graphical representation of a process showing plotted values of a representative statistic of a selected characteristic (for instance, the individual value, mean, range or standard deviation), a centre line, and one or more control lines. The control line(s) and centre line are used as a basis for judging the stability ol the process, namely, whether or not the process is in a state of statistical control. Control lines are derived from the actual performance of the process and are not to be confused with specified limits or specified tolerances.
Shewhart control charts provide a common language for communicating technical information on the performance of a process. Control charts are effective tools in understanding process behaviour. They distinguish between special and common cause variation. When no special cause is present, the process is said to be in a state of statistical control.
When a process Is In statistical control, its capability Is predictable and can be assessed. Reducing common cause variation and improving process targeting can enhance process capability.
Potentially, the control chart has wide applicability throughout any organization.
4.2 How to select the correct type of Shewhart control chart for measured data generally
The procedure for selecting a Shewhart type measured data control chart is as follows.
a) If the characteristic to be monitored is ongoing with a targeted constant aim and process spread, refer to ISO 7870-2.
b) If the characteristics do not have a constant aim or process spread. and the sample size is limited to one, see i3.BS ISO 7870-8 pdf free download.
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