nP Chart Calculations

The nP chart measures the number of nonconforming items in an inspection lot. It is identical to P chart except that the actual number of nonconforming items, rather than their proportion of the sample, is plotted. The inspection sample sizes must be equal in this case.

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 :

Process Average Number Non conforming,


Sigma (Standard deviation)


Center Line and Control limits

Center Line ( CLNP )

Upper Control Limit (UCLNP)


Lower Control Limit (LCLNP)

MAX(0,(CLNP – 3 SNP))

More information

SPC chart calculations.