Quality and Reliability Methods - SAS

Chapter 5 Shewhart Control Charts 81
Moving Average Charts
Although the points for X - and S-charts are the same as the Individual on Group Means and Individual on
Group Std Devs charts, the limits are different because they are computed as Individual charts.
Another way to generate the presummarized charts, with the Coating.jmp data table,
1. Choose Tables > Summary.
2. Assign Sample as the Group variable, then Mean(Weight) and Std Dev(Weight) as Statistics.
3. Click OK.
4. Select Analyze > Quality And Process > Control Chart > IR.
5. Select Mean(Weight) and Std Dev(Weight) as Process variables.
6. Click OK.
The resulting charts match the presummarized charts.
When using Presummarize charts, you can select either On Group Means or On Group Std Devs or both.
Each option will create two charts (an Individual Measurement, also known as an X chart, and a Moving
Range chart) if both IR chart types are selected.
The On Group Means options compute each sample mean and then plot the means and create an
Individual Measurement and a Moving Range chart on the means.
The On Group Std Devs options compute each sample standard deviation and plot the standard deviations
as individual points. Individual Measurement and Moving Range charts for the standard deviations then
appear. Note that as a dispersion chart, the only Warnings option available for an Individual on Group Std
Dev chart is Test Beyond Limits.
Moving Average Charts
The control charts previously discussed plot each point based on information from a single subgroup
sample. The Moving Average chart is different from other types because each point combines information
from the current sample and from past samples. As a result, the Moving Average chart is more sensitive to
small shifts in the process average. On the other hand, it is more difficult to interpret patterns of points on a
Moving Average chart because consecutive moving averages can be highly correlated (Nelson 1983).
In a Moving Average chart, the quantities that are averaged can be individual observations instead of
subgroup means. However, a Moving Average chart for individual measurements is not the same as a control
(Shewhart) chart for individual measurements or moving ranges with individual measurements plotted.
Uniformly Weighted Moving Average (UWMA) Charts
Each point on a Uniformly Weighted Moving Average (UWMA) chart, also called a Moving Average chart,
is the average of the w most recent subgroup means, including the present subgroup mean. When you
obtain a new subgroup sample, the next moving average is computed by dropping the oldest of the previous
w subgroup means and including the newest subgroup mean. The constant, w, is called the span of the
moving average, and indicates how many subgroups to include to form the moving average. The larger the
span (w), the smoother the UWMA line, and the less it reflects the magnitude of shifts. This means that
larger values of w guard against smaller shifts.