The P chart measures the proportion of nonconforming (discrepant or defective) items in a group of items being inspected. Proportion is defined as the ratio of the number of nonconforming items in a population to the total number of items in that population. Sample sizes need not be equal.
Data Available
N = Number of units produced in each sample ( subgroup size )
R(1..K) = Number of units rejected in each subgroup
K = Inspection Quantity
To be calculated:
Average Proportion Non conforming,
=
Sigma ( standard deviation)
SP =
Center Line and Control Limits
The control limits are the process average plus or minus an allowance for the variation that could be expected if the process where in statistical control, given the subgroup size. For the study period of k subgroups, calculate the upper & lower control limits:
Center Line ( CLP ) 

Upper Control Limit (UCLP) 
CLP + 3 SP 
Lower Control Limit (LCLP) 
MAX(0,(CLP – 3 SP)) 
SPC chart calculations. 