Np-chart

In industrial statistics, the np-chart is a type of control chart that is very similar to the p-chart except that the statistic being plotted is a number count rather than a sample proportion of items. For example, an np-chart often shows the number of nonconforming items in each sample. Since we are counting failures or successes, clearly the appropriate data for np-charts need to be attribute data. The subgroup size must be constant, as comparisons of counts would otherwise be meaningless.

np-chart


 * 1) The "np" stands for the number of nonforming items, which can be expressed as n (sample size) times p (proportion of nonconforming items)
 * 2) Need a good definition of nonconforming items – usually a categorical definition
 * 3) Subgroup size must be constant
 * 4) Normally need large subgroups – can even be up to total for the period

Control limits for the np-chart are calculated on the basis of the binomial distribution.

The control limits for this chart type can be determined by the formula: $$n\bar p \pm 3\sqrt{\frac{n\bar p(1-\bar p)}{n}}$$