What is an NP chart?

Attribute Chart (np chart) is the control chart for defects (d-chart) also known by the name of Attribute charts. The chart is used to monitor the number of defective or non-conforming items during the measurement process. It uses binary distribution for measuring the number of non-conforming or defective units in a sampling. The np chart looks very similar to the P chart. The p charts plot the percentage of defective items while the np chart plots the number of items.

The np chart plots the number and type of defects on the y-axis. The centerline (np), is the total number of non-conforming or defective units divided by the number of lots.

Selecting Control Chart

The Control Chart can be used to track the evolution of a process. The control chart has three lines: a central line representing the average, a top line representing the upper limit of the control, and a bottom line representing the lower limit. The control limits are -3s away from the centerline.

Control chart mapping is not complete without a suitable control table. Otherwise, the control limits will be inaccurate.

X- and R-charts can be used to represent measurable quantities, such as weight, height, and length. Attribute Control Charts are for attribute data. The data is used to count the number of defects per unit. The number of tubes that failed in a shop, for example. For attributes, unlike variable charts, there is only one chart.

When and why do you use an NP chart?

This chart allows you to evaluate trends and patterns of binary events over time. This chart type requires the same sample size for each subgroup and calculates control limits using the binary distribution.

Four types of control charts exist for attribute data. The chart shows the defect rate per unit. The chart shows the number of defects. The chart shows the percentage of defective items. The np charts reflect integers rather than proportions. The p-chart can be used to apply the np chart.