np-chart

{{Lowercase}}

{{Infobox control chart

| name = np-chart

| proposer = Walter A. Shewhart

| subgroupsize = n > 1

| measurementtype = Number nonconforming per unit

| qualitycharacteristictype = Attributes data

| distribution = Binomial distribution

| sizeofshift = ≥ 1.5σ

| meanchart = Np control chart.svg

| meancenter =

| meanlimits = $n\; \backslash bar\; p\; \backslash pm\; 3\backslash sqrt\{n\; \backslash bar\; p(1-\; \backslash bar\; p)\}$

| meanstatistic =

}}

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 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.{{cite book | last = Montgomery | first = Douglas | title = Introduction to Statistical Quality Control | publisher = John Wiley & Sons, Inc. | year = 2005 | location = Hoboken, New Jersey | pages = 279 | url = http://www.eas.asu.edu/~masmlab/montgomery/ | isbn = 978-0-471-65631-9 | oclc = 56729567 | url-status = dead | archiveurl = https://web.archive.org/web/20080620095346/http://www.eas.asu.edu/~masmlab/montgomery/ | archivedate = 2008-06-20 }}

The np-chart differs from the p-chart in only the three following aspects:

1. The control limits are $n\backslash bar\; p\; \backslash pm\; 3\backslash sqrt\{n\backslash bar\; p(1-\backslash bar\; p)\}$, where n is the sample size and $\backslash bar\; p$ is the estimate of the long-term process mean established during control-chart setup.
2. The number nonconforming (np), rather than the fraction nonconforming (p), is plotted against the control limits.
3. The sample size, $n$, is constant.