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p Control charts are attribute type charts used to plot units nonconforming when the samples are not of equal size. This is often when samples form a natural grouping - for example the number of treatments in a hospital in a week.
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Upper Control Limit
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Lower Control Limit
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Note that the control limits are dynamic, they depend on the size of the sample.
If the samples sizes are of equal size an np control chart can be used.
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