Quality and Reliability Methods - SAS

76 Shewhart Control Charts Chapter 5
Shewhart Control Charts for Variables
Run Charts
Run charts display a column of data as a connected series of points. The following example is a Run chart
for the Weight variable from Coating.jmp.
Figure 5.4 Run Chart
When you select the Show Center Line option in the Run Chart drop-down, a line is drawn through the
center value of the column. The center line is determined by the Use Median setting of the platform
drop-down. When Use Median is selected, the median is used as the center line. Otherwise, the mean is
used. When saving limits to a file, both the overall mean and median are saved.
Run charts can also plot the group means when the Sample Label role is used, either on the dialog or
through a script.
Individual Measurement Charts
Individual Measurement Chart Type displays individual measurements. Individual Measurement charts are
appropriate when only one measurement is available for each subgroup sample.
Moving Range Chart Type displays moving ranges of two or more successive measurements. Moving ranges
are computed for the number of consecutive measurements that you enter in the Range Span box. The
default range span is 2. Because moving ranges are correlated, these charts should be interpreted with care.
Example: Individual Measurement and Moving Range Charts
The Pickles.jmp data in the Quality Control sample data folder contains the acid content for vats of pickles.
Because the pickles are sensitive to acidity and produced in large vats, high acidity ruins an entire pickle vat.
The acidity in four vats is measured each day at 1, 2, and 3 PM. The data table records day, time, and
acidity measurements. The dialog in Figure 5.5 creates Individual Measurement and Moving Range charts
with date labels on the horizontal axis.