Page 119 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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3.3.5 Attribute control charts limit calculations
All attribute control charts follow the same three sigma control lim-
it away from the centerline methodology of the variable control
charts:
Control limits for attribute charts = centerline ± 3 s
For constant samples (C or nP charts)
For Poisson distribution:
Centerline = Poisson average or c (3.8)

s = = c for Poisson standard deviation

and
CL c = c ± 3 · c

np
n p = (3.9)
k
For binomial distribution:
Centerline = Binomial average n p

n
n
s = n p · ( 1 – p )

CL np = np ± 3 · n p · ( 1 – p ) (3.10)
n
n

where
c = number of defects in a unit

np = number of defectives found in each constant sample n

n is the number of units in sample
k is the number of samples
For changing sample sizes (U or P charts)
For Poisson distribution:
Centerline = Poisson average number of defects in a sample u

c
u =

n
u

s = = for Poisson standard deviation
n
and
u

CL u = u ± 3 · n ; (3.11)

For binomial distribution: