Probability Applications

266 Chapter 12. Control Charts for Statistical Process Control
Figure 12.1. Shewhart X bar chart for the log cabin problem (first try).
Figure 12.2. The R chart for the log cabin problem (first try}.
There are several things that need to be explained about these charts. First, in Figure
12.1, the set averages, the grand average (the average of the set averages), and the upper
and lower control limits (UCL^ andLCL*, respectively) are plotted for each set for a given
run length. In this case the run length is 20 sets. Second, in Figure 12.2, the set ranges,
the range average, and the upper range control limit, UCL r , are plotted for each set for the
run. (When the set size is 6 or smaller, no lower range control limit can be computed.) A