A First Course in Quality Engineering, 2nd Edition

Quality in Production—Process Control II 337

CV

m

S

T

i

i

i

=













∑

1

2

where S

i 

/T

i

is the estimate of the CV from individual subgroups� Then,

according to Farnum (1992):

X T

CV T n

N

i

i

i

−

( )

≈

(

)

0 1,

and

P

X T

CV T n

P

CV T

n

X T

CV T

n

i

i

i

i

i

i

i

− ≤

−

( )

≤













=

−

( )

≤ − ≤

( )





3 3 0 9973

3 3

.









=

−

( )

≤

−

≤

( )













=

−

( )

≤

0 9973

3 3 0 9973

3

.

.P CV n

X T

T

CV n

P CV n

X

i

i

i

ii

i

i

i

T

CV n

P CV n

X

T

CV n

− ≤

( )













=

−

( )

≤ ≤ +

( )













=

1 3 0 9973

1 3 1 3 0

.

.99973

5 10

Sample no.

15 20

LCL = –0.00287

CL

UCL = 0.00287

–0.003

–0.002

–0.001

0.000

X – T

0.001

0.002

0.003

FIGURE 5.16

Example of a DNOM control chart�

338 A First Course in Quality Engineering

Thus, if we plot the statistic X

i 

/T

i

, which ...