Quality and Reliability Methods - SAS

90 Shewhart Control Charts Chapter 5
Shewhart Control Charts for Attributes
• Click OK.
Control Chart(Sample Label( :Box), Sample Size( :Box Size), K Sigma(3), Chart
Col( :Name("# Defects"), C));
Figure 5.15 C-Chart
Control Limits on C-Charts
The lower and upper control limits, LCL and UCL, are computed as follows.
LCL = max( n i
u – k n i
u,
0)
UCL =
n i
u + k n i
u
The limits vary with n i .
u is the expected number of nonconformities per unit produced by process
u i is the number of nonconformities per unit in the i th subgroup. In general, u i = c i /n i .
c i is the total number of nonconformities in the i th subgroup
n i is the number of inspection units in the i th subgroup
u is the average number of nonconformities per unit taken across subgroups. The quantity u
computed as a weighted average
n 1
u 1
+ … + n N
u N
c
u -------------------------------------------- 1
+ … + c N
= = -------------------------------
n 1
+ … + n N
n 1
+ … + n N
is
N is the number of subgroups