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362 Part V: Appendices
The p Chart: Control Chart for Fraction Nonconforming Units. First enter
the data (number of nonconforming) in one column, say C1, of the Worksheet
window. Then from the command menu select Stat > Attributes Charts > P. The
dialog box titled P Chart shown in Figure B.19 appears immediately. We illustrate
the construction of a p chart with Example B.15.
The p Chart: Control Chart for Fraction Nonconforming Units with Variable
Sample Sizes. The procedure for constructing a p chart with variable sample size
is the same as for p chart with fixed sample size. Except in this case we enter
the data on number of nonconforming in column C1 and the corresponding
sample sizes in column C2, and in step 6 instead of entering the sample size in the
box next to Subgroup Sizes we enter C2.
EXAMPLE B.15
A semiconductor manufacturer tracks the number of nonconforming computer chips
produced each day. A team of Six Sigma Green Belt engineers wants to improve the
overall quality by reducing the fraction of nonconforming computer chips. To achieve
this goal the team decided to set up a p chart, and in order to do so they decided to
inspect a sample of 1000 chips each day over a period of 30 days. Table 19.5, page 279
gives the number of nonconforming chips out of 1000 inspected chips each day during
the study period of 30 days.
Solution:
1. Enter all the data (number of nonconforming given in Table 19.5 in
Chapter 19) in column C1 of the Worksheet window. Click on Stat from
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 on P in the Attributes Charts command menu. The dialog box titled P Chart
shown in Figure B.19 appears immediately.
5. Enter C1 in the box under variables.
6. Enter the sample size in the box next to Subgroup Sizes.
7. In the dialog box titled P Chart there are several options available such as Scale
and Labels. Thus, for instance, if you select the Label option then a new dialog
box will appear where you can enter the title of the p chart and any footnotes
that you would like to see on the output of the p chart and then click OK. By
default the title will be such as P Chart for C1 or P Chart for “name of the variable”
if you have given such name in column C1 of the data window. Use the option
P Chart Options, for example, if you want to specify the values of fraction of
nonconforming in the population instead of estimating it using the given data.
Then, click OK in the P Chart dialog box. The desired p control chart will appear
in the Session window. Thus, in our example the output of the p chart is as
shown in Figure 19.10 of Chapter 19, page 280.
Continued