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364 Part V: Appendices
menu select Stat > Attributes Charts > C. The dialog box titled C Chart shown in
Figure B.20 appears immediately. We illustrate the construction of a c chart with
Example B.16.
The u Chart. The u chart is essentially the c chart except that the u chart is always
based on the number of nonconformities per inspection unit. In other words, the
actual sample size may be other than one or may vary, but the control limits of the
u chart are always determined based on one inspection unit. Thus, for example,
if n is constant then one can use either a c chart or a u chart. We illustrate the construction
of a u chart with Example B.17.
The u Chart—Variable Sample Sizes. The procedure for constructing a u chart
with variable sample size is the same as for the u chart with fixed sample size.
EXAMPLE B.16
A paper mill has detected that almost 90 percent of rejected paper rolls are due to
nonconformities of two types, that is, holes and wrinkles in the paper. The Six Sigma
Green Belt team in the mill decided to set up control charts to reduce or eliminate these
nonconformities. To set up control charts the team decided to collect some data by
taking a random sample of five rolls each day for 30 days and counting the number of
nonconformities (holes and wrinkles) in each sample. The data are shown in Table 19.7,
page 287. Set up a c control chart using these data.
Solution:
1. Enter all the data (number of nonconformities given in Table 19.7 in
Chapter 19) in column C1 of the Worksheet window. Click on Stat in the
command menu.
2. Select Control Charts in the pull-down menu under the Stat command menu.
3. Select Attributes Charts from the Control Charts command menu.
4. Click C in the Attributes Charts command menu. The dialog box entitled C Chart
shown in Figure B.20 appears immediately.
5. Enter C1 in the box under Variables.
6. In the dialog box entitled C Chart there are several options available such as Scale
and Labels. Thus, for instance, if you select the Label option a new dialog box will
appear where you can enter the title of the c chart and any footnotes that you
would like to see on the output of the c chart and then click OK. By default the
title will be such as C Chart for C1 or C Chart for “name of the variable” if you
have given such name (say nonconformities) in column C1 of the data window.
Use the option C Chart Options, for example, if you want to specify the mean
value of nonconformities in the population instead of estimating it using the given
data. Then, click OK in the C Chart dialog box. The desired c control chart will
appear in the Session window. Thus, in our example the output of the c chart is as
shown in Figure 19.13 of Chapter 19, page 287.
Continued