Quality and Reliability Methods - SAS

102 Cumulative Sum Control Charts Chapter 6
Cumulative Sum (Cusum) Charts
A shift or out-of-control condition is signaled at a time t if one or more of the points plotted up to the time
t cross an arm of the V-mask. An upward shift is signaled by points crossing the lower arm, and a downward
shift is signaled by points crossing the upper arm. The time at which the shift occurred corresponds to the
time at which a distinct change is observed in the slope of the plotted points.
Automatic Updating
The cusum chart automatically updates when you add new samples:
1. Start from the chart in Figure 6.6.
2. Open the Oil2 Cusum.jmp sample data table.
3. Copy rows 49 through 60. Paste them into the end of the Oil1 Cusum.jmp sample data table.
Notice that the graph in the CUSUM of weight report updates automatically. See Figure 6.7.
Figure 6.7 Updated Cusum Chart for the Oil1 Cusum.jmp Data
You can move the origin of the V-mask by using the grabber tool to click a point. The center line and
V-mask adjust to reflect the process condition at that point.
Example 2. One-Sided Cusum Chart with No V-mask
Consider the data used in “Example 1. Two-Sided Cusum Chart with V-mask” on page 100, where the
machine fills 8-ounce cans of engine oil. Consider also that the manufacturer is now concerned about
significant over-filling in order to cut costs, and not so concerned about under-filling. A one-sided Cusum
Chart can be used to identify data approaching or exceeding the side of interest. Anything 0.25 ounces
beyond the mean of 8.1 is considered a problem. To do this example,
• Open the Oil1 Cusum.jmp sample data table.
• Choose the Analyze > Quality And Process > Control Chart > CUSUM command.
• Deselect the Two Sided check box.
• Specify weight as the Process variable.