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In Statistical Process Control subgroups are collected from the process and their statistics (typically the mean and range or mean and standard deviation) are plotted on control charts.
The subgroups should be collected in a way that minimizes the opportunities for special causes to occur within a subgroup. The items in a subgroup should be collected closely together, and can even be consecutive items. Items within a subgroup should not be collected across a shift change, or across any other process change that may be associated with a special cause.
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