Whether the output of a process or service conforms

whether the output of a process or service conforms to the product design. SPC employs
a number of techniques to measure the current quality of products and detect whether the
process itself has changed in a way that affects the quality. Some of these methods are
control charts for variables (R-chart and
x
-chart), control charts for attributes (p-chart
and c-chart), process capability ratio, and process capability index.
The R-chart and
x
-chart are used to monitor the mean and variability of process
distribution. The p-chart is used to control the proportion of defective products or services
generated by the process. The c-chart is used to control the number of defects when more
than one defect could be present in a product or service. The process capability ratio is
equal to the ratio of the tolerance width and 6 standard deviations. The process capability
index measures the potential for a process to generate defective outputs relative to either
upper or lower specifications. The last two measures indicate the ability of a process to
meet the design specifications for a product or service.
The aim of this project is to build a decision support system that enables the managers to
use SPC methods to determine whether the quality of the products produced meets the
specifications and to identify what causes the problem in the case when these
specifications are not met. To learn more about SPC techniques, the reader is referred to
Krajewski and Ritzman (2002).
Excel Spreadsheets
1. The following spreadsheet presents the factors for calculating the three-sigma limits
for the R-chart and
x
-chart.
User Interface
1. Build a welcome form.
2. Build a form that includes a frame that has four option buttons. The option buttons
enable the user to select one of the following SPC techniques: control charts for
Sample Size
(n)
Factor for UCL and LCL
for
x
-charts
(A
2
)
Factor for LCL for
R-charts
(D
3
)
Factor for UCL for
R-charts
(D
4
)
2 1.880 0.000 3.267
3 1.023 0.000 2.575
4 0.729 0.000 2.282
5 0.577 0.000 2.115
6 0.483 0.000 2.004
7 0.419 0.076 1.924
8 0.373 0.136 1.864
9 0.337 0.184 1.816
10 0.308 0.223 1.777
15

Case Study 15 Statistical Process Control
43
variables, control charts for attributes (the p-chart), control charts for attributes (the c-
chart), and the process capability ratio/index. Insert a command button that, when
clicked on, submits the user’s choice.
a. If the user selects the first option button, display two text boxes where the user
can type in the sample size and number of replications. Insert a command button
that, when clicked –on, displays a table that has as many rows as the number of
replications and as many columns as the sample size. The user types in the
results from the observations in this table. Insert a command button that, when
clicked on, does the following: submits the data entered by the user;
calculates
R
,
x
, UCL
R
and LCL
R
, UCL
x
and LCL
x
;creates the R-chart and
x
-chart; and plots the observations on these charts.