Control Charts

 

Control Chart 1

When plotted on a chart this looks like the one on the right. We can immediately see that the process is in control based on the three sigma rule, but is that sufficient? Shewhart later modified his control scheme to account for other types of non-normal distribution such as time series trends, process shifts and other special causes of variability that did not violate the three sigma rule.

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Shewhart's Modification

Control Charts with ± 1 and 2 sigma lines

Control Chart for Moving Ranges

Shewhart’s modification added a set of control rules, violation of any of which would constitute an out of control condition.

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