Data Analysis with Microsoft Excel: Updated for Office 2007

Chapter 12 Quality Control 503

Use the information in the Coats workbook to determine whether the

range of coating weights is in control.

To create a range chart of the weight values:

1 Return to the Coating Data worksheet.

2 Click QC Charts from the StatPlus menu and click Range Chart.

3 Select Weight as your Data Values variable and Time as the Sub-

group variable.

4 Verify that the Sigma Known checkbox is unselected.

5 Direct the output to a new chart sheet named Range Chart. Click

OK twice.

Figure 12-10
The range
control chart

value is

not within

control limits

Each point on the range chart represents the range within each subgroup.

The average subgroup range is 3.25, with the control limits going from 0 to

10.62. According to the range chart shown in Figure 12-10, only the 27th ob-

servation has an out-of-control value. The special cause should be identifi ed

if possible. However, in discussing the problem with the operator, sometimes

you might not be able to determine a special cause. This does not necessarily

mean that no special cause exists; it could mean instead that you are unable

to determine what the cause is in this instance. It is also possible that there

really is no special cause. However, because you are constructing control

charts with the width of about 3 standard deviations, an out-of-control value

is unlikely unless there is something wrong with the process.