In
statistical quality control
Statistical process control (SPC) or statistical quality control (SQC) is the application of statistical methods to monitor and control the quality of a production process. This helps to ensure that the process operates efficiently, producing ...
, the np-chart is a type of
control chart
Control charts is a graph used in production control to determine whether quality and manufacturing processes are being controlled under stable conditions. (ISO 7870-1)
The hourly status is arranged on the graph, and the occurrence of abnormalit ...
used to monitor the number of
nonconforming units in a
sample. It is an adaptation of the
p-chart 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
References
{{DEFAULTSORT:Np-Chart
Quality control tools
Statistical charts and diagrams