The p chart:
For each sample (subgroup) the failure proportion (p) is calculated and charted in the control chart. The failure proportion is calculated as shown below:
Where:
NF=number of failures in the sample
n=sample size (number inspected in sub group)
Construction of control limits is done as follows:
Where:
And:
TNF=Total Number of Failures in all the samples inspected
TNI=Total Number Inspected (the sum of all samples).
The np chart:
For each sample the number of failures (np=the number of non-conforming units) is counted and charted in the control chart. Construction of the control limits is done as follows: