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

# The "np" stands for the number of nonforming items, which can be expressed as n (sample size) times p (proportion of nonconforming items)
# Need a good definition of nonconforming items – usually a categorical definition
# Subgroup size must be constant
# 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 and an approximation based on the central limit theorem.

The control limits for this chart type can be determined by the formula: