Hansen John P
Group Health Cooperative of South Central Wisconsin, Madison, USA.
J Healthc Qual. 2005 Jul-Aug;27(4):45-52. doi: 10.1111/j.1945-1474.2005.tb00567.x.
A p control chart is used to monitor a proportion from a binomial variable that reflects the output of a process. The p control chart is a graph that includes serial sample proportions (p) as the variables of interest, a centerline that represents the overall proportion of the samples (p), and upper control limits (UCLs) and Lower control Limits (LCLs) that represent three standard errors (SEp) above and below the centerline. We use a p control chart to estimate with 99.7% confidence that the population proportion of an output variable was within the interval defined by the UCLs and LCLs during a period of baseline monitoring. We further assume that if the process remains stable, the values of future population proportions will remain between the established control limits. An np control chart simply plots the numerators of the sample proportions as the variables of interest. A u control chart is used to monitor ratios. In u control charts, ratios are quantities in which the numerator can have one or more occurrences in reference to the denominator. A c control chart is analogous to an np control chart in that it graphs the numerators from the ratios.
