Shewhart Control Chart
From Open Risk Manual
Definition
The Shewhart Control Chart is a core tool proposed by Shewhart as an element of Statistical Process Control. It aims to provide an assessment of whether a Process is in a Stable State.
Diagram
Methodology
A control chart is made by plotting measurements of a Statistic (capturing e.g. quality characteristiscs, system conditions etc.) as a time ordered series of samples or subgroups[1]
Measurements within a subgroup (also called "rational subgroup") are assumed to vary only due to random causes. Thus variability between different subgroups prompts for seeking an Assignable Cause
The limits around average measurement within a subgroup (typically two limits, the Upper Control Limit and the Lower Control Limit can be set according to various methods:
- Using the range (difference between maximum and minimum observations)
- Using the empirical variance (e.g. 3 sigma level)
See Also
References
- ↑ Encyclopedic Dictionary of Mathematics
