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)) |
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SPC chart calculations. |
