In
statistical quality control, the np-chart is a type of
control chart used to monitor the number of
nonconforming units in a
sample. It is an adaptation of the
p-chart
In statistical quality control, the p-chart is a type of control chart used to monitor the proportion of nonconforming units in a sample, where the sample proportion nonconforming is defined as the ratio of the number of nonconforming units to ...
and used in situations where personnel find it easier to interpret process performance in terms of concrete numbers of units rather than the somewhat more abstract proportion.
The np-chart differs from the p-chart in only the three following aspects:
#The control limits are
, where n is the sample size and
is the estimate of the long-term process mean established during control-chart setup.
#The number nonconforming (np), rather than the fraction nonconforming (p), is plotted against the control limits.
#The sample size,
, is constant.
See also
*
p-chart
In statistical quality control, the p-chart is a type of control chart used to monitor the proportion of nonconforming units in a sample, where the sample proportion nonconforming is defined as the ratio of the number of nonconforming units to ...
References
{{DEFAULTSORT:Np-Chart
Quality control tools
Statistical charts and diagrams