16.1. TRADITIONAL CONTROL CHARTS
Traditional control charts have two graphs.
The top graph is based on the process average, with statistically based control rules and limits telling the operator when the process is in or out of control. This graph cannot have any reference to the product tolerance because it displays product averages. By definition, a tolerance is for individual parts and is meaningless on averages. When tolerances are displayed on traditional control charts, which people sometimes try to do, they make no sense and can give an erroneous message that a process is running well when it isn't.
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