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 mo ...

, the c-chart is a type of

control chart Control charts are graphical plots 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 ...

used to monitor "count"-type data, typically total number of nonconformities per unit. 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 statistical quality control, the p-chart is a type of control chart used to monitor the proportion of nonconforming units in a sample, where the sample proportion nonconforming is defined as the ratio of the number of nonconforming units to ...

in that it accounts for the possibility of more than one nonconformity per inspection unit, and that (unlike the

and u-chart) it requires a fixed sample size. The p-chart models "pass"/"fail"-type inspection only, while the c-chart (and u-chart) give the ability to distinguish between (for example) 2 items which fail inspection because of one fault each and the same two items failing inspection with 5 faults each; in the former case, the p-chart will show two non-conformant items, while the c-chart will show 10 faults. 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 Injection moulding (U.S. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals (for ...

casting Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or ...

processes *Monitoring the number of discrete components that must be re-

soldered Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...

per

printed circuit board A printed circuit board (PCB), also called printed wiring board (PWB), is a Lamination, laminated sandwich structure of electrical conduction, conductive and Insulator (electricity), insulating layers, each with a pattern of traces, planes ...

*Monitoring the number of product returns per day The

Poisson distribution In probability theory and statistics, the Poisson distribution () is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time if these events occur with a known const ...

is the basis for the chart and requires the following assumptions: *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

\bar c \pm 3\sqrt

where

\bar c

is the estimate of the long-term process mean established during control-chart setup.

References

{{DEFAULTSORT:C-Chart Quality control tools Statistical charts and diagrams

See also

References