A p-Chart is used to analyze the ratio of the proportion defective in a sample to each sample.
The upper control limit and lower control limit for a p-Chart are defined as:
UCL = ps + zσ
LCL = ps - zσ
where
z is the number of standard deviations
ps is the proportion defective
σ is the standard deviation of the sample proportion
σ can be obtained by:
σ = √(ps * (1 - ps) / n), where n is the sample size
thus UCL and LCL can be rewritten as:
UCL = ps + z * √(ps * (1 - ps) / n)
LCL = ps - z * √(ps * (1 - ps) / n)