100-Metodos-de-Qualidade-Total

162 100 METHODS FOR TOTAL QUALITY MANAGEMENT
the process. C charts give a way to do this for the number of defects or
problems with a sample size that is constant.
Example
An organization is experiencing problems with the number of errors or
mis-codings of invoices and a C chart was used to monitor the error rate. A
constant number of 80 invoices was sampled each week with the following
results (see Figure 1).
Lot no.
No. of errors
1 5
2 3
3 0
4 4
5 7
6 0
7 2
8 9
9 5
10 6
11 0
12 8
13 5
14 0
15 6
16 2
17 7
18 0
19 7
20 4
For the data above the calculations were as follows:
Reference
n = 80 m = 20
k
CL C C 80
= =
=
m 20
K = 3vC = 3\1'4= 6
UCL = C + K = 10
LCL C
= - K = 0
D.M. Amsden, H.E. Butler and R.T. Amsden (1991) SPC Simplified for Services. London:
Chapman and Hall.
= 4