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Ignore my previous message. See the correct explanation below:
A np-chart is useful when the number of units or sample size in each subgroup is constant as interpretation is easier than a p-chart. For data with different subgroup sizes the center line and control limits both vary making interpretation difficult. In this case, you should use a p-chart which has a constant center line but varying control limits.
We have the same difference between C-Chart and U-Chart.
The u-chart differs from the c-chart in that it accounts for the possibility that the number or size of inspection units for which defectives are to be counted may vary. In other words, for plotting "Defects", you should use the u-control-chart if the subgroup size changes or is different from subgroup to subgroup (instead of the c-control-chart). Note that if the subgroup size changes from subgroup to subgroup, the control limits will vary as well, and you will end up with different control-limits for each subgroup on the u-control-chart.
But do not forget that np-chart and p-chart are used for plotting "Defectives", whereas c-chart and u-chart are used for plotting "Defects".
Only in P-Chart to calculate the upper and lower control limits the equation is divided by the sample size average (n-bar), and that is why the sample size affects the control limits.
UCL = p-bar + 3 x SquareRootOf(p-bar(1 - p-bar) / Sample-Size-Average)
Please ignore the message above, it does not make any sense at all.
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