vdoc

Chapter 19: B. Statistical Process Control 271
Step 3. Calculate the control limits for the X – chart:
sˆ
UCL = x + 3
n
s
= x + 3
c n
4
= x+
A s
CL = x
sˆ
LCL = x 3
n
3
s
= x 3
c n
4
= x
A s
3
(19.23)
(19.24)
(19.25)
where the values of A 3 and c 4 for various sample sizes are given from
Appendix E.
Step 4. Calculate the control limits for the S chart:
UCL = s + 3sˆ
s
= s + 3 1
c
c
s
4
2
4
= +
1 3 1 (19.26)
2
1 c
c
s
4
4
= B s 4
CL = s
(19.27)
Part IV.B.3
LCL = s 3sˆ
s
s
2
= s 3 1c4
c4
=
1 3 1 (19.28)
2
1 c
c
s
4
4
= B s 3
We illustrate the development of X – and S control charts with the following
example.