- C-chart
In industrial
statistics , the c-chart is a type ofcontrol 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 downassignable cause s.Examples of processes suitable for monitoring with a c-chart include:
*Monitoring the number of voids per inspection unit ininjection molding orcasting processes
*Monitoring the number of discrete components that must be re-soldered perprinted circuit board
*Monitoring the number of product returns per dayThe
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 sampleThe control limits for this chart type are ar c pm 3sqrt{ar c} where ar c is the estimate of the long-term process mean established during control-chart setup.
References
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