Data Analysis with Microsoft Excel: Updated for Office 2007

512 Statistical Methods

The chart shown in Figure 12-16 gives the values of the individual

observations (not the moving averages) plotted alongside the upper and

lower control limits. No values fall outside the control limits; this leads us

to conclude that the process is in control. However, the last eight observa-

tions are all either above or near the center line; this might indicate a pro-

cess going out of control toward the end of the process. This is something

that should be investigated further.

We should also plot the moving range chart, to see whether there is any

evidence in that plot of an out-of-control process.

To create a moving range chart for the steel samples:

1 Click QC Charts from the StatPlus menu and then click Moving

Range Chart.

2 Select Strength for the Data Values variable.

3 Send the output to a new chart sheet named MR-Chart.

4 Click OK. Excel generates the chart shown in Figure 12-17.

trend in the

moving range

chart indicates

a process not

in control

Figure 12-17

The moving

range chart

for tensile

strength

samples

The chart in Figure 12-17 shows additional indications of a process that

is not in control. The last seven values all fall below the center line, and

there appears to be a generally downward trend to the ranges from the sixth

observation on. We would conclude that there is suffi cient evidence to war-

rant further investigation and analysis.

You can save and close the Strength Control Chart workbook now.