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)
SNP =
Center Line and Control limits
Center Line ( CLNP ) 

Upper Control Limit (UCLNP) 
CLNP + 3 SNP 
Lower Control Limit (LCLNP) 
MAX(0,(CLNP – 3 SNP)) 
SPC chart calculations. 