U Chart Calculations

The U chart measures the number of non-conformities per inspection unit in subgroups, which can have varying sample sizes. It is similar to C chart except that the number of non-conformities is expressed on a per unit basis.

Data Available

N = Subgroup size

T(1..K) = Total produced

K = Inspection Quantity

D(1..K) = Total Defects in sample

To be calculated:

Process Average Non conformities Per Unit,

  =      U(1..K) where UI = DI / TI

Sigma (Standard deviation )

SU =

Center Line and Control limits :

Center Line ( CLU )

Upper Control Limit (UCLU)

CLU + 3 SU

Lower Control Limit (LCLU)

MAX(0,(CLU – 3 SU))

More information

SPC chart calculations.