C-chart

In industrial statistics, the c-chart is a type of control chart used to monitor "count"-type data, typically total number of nonconformities per unit.cite web|url=http://www.itl.nist.gov/div898/handbook/pmc/section3/pmc331.htm|title=Counts Control Charts|accessdate=2008-08-23|work= [http://www.itl.nist.gov/div898/handbook/index.htm NIST/Sematech Engineering Statistics Handbook] |publisher=National Institute of Standards and Technology It is also occasionally used to monitor the total number of events occurring in a given unit of time.

The c-chart differs from the p-chart in that it accounts for the possibility of more than one nonconformity per inspection unit. The p-chart models "pass"/"fail"-type inspection only. Nonconformities may also be tracked by type or location which can prove helpful in tracking down assignable causes.

Examples of processes suitable for monitoring with a c-chart include:
*Monitoring the number of voids per inspection unit in injection molding or casting processes
*Monitoring the number of discrete components that must be re-soldered per printed circuit board
*Monitoring the number of product returns per day

The Poisson distribution is the basis for the chart and requires the following assumptionscite book | last = Montgomery | first = Douglas | title = Introduction to Statistical Quality Control | publisher = John Wiley & Sons, Inc. | date = 2004 | location = New York, New York | pages = 289 | url = http://www.eas.asu.edu/~masmlab/montgomery/ | isbn = 9780471656319 | oclc = 56729567:
*The number of opportunities or potential locations for nonconformities is very large
*The probability of nonconformity at any location is small and constant
*The inspection procedure is same for each sample and is carried out consistently from sample to sample

The control limits for this chart type are where is the estimate of the long-term process mean established during control-chart setup.

References