Example of two one-sided CUSUM charts
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Suppose you work at a car assembly plant in a department that assembles engines. In an operating engine, parts of the crankshaft move up and down a certain distance from an ideal baseline position. AtoBDist is the distance (in mm) from the actual (A) position of a point on the crankshaft to the baseline (B) position.

To monitor the process, you collect a subgroup of 5 parts each afternoon and measure the AtoBDist for the parts. After 13 days, you decide that it would be useful to sample the process during the morning shift as well. You decide to collect two subgroups of parts each day, one during the morning shift and one during the afternoon shift.

You already displayed an X chart and an R chart of this data. On the X chart, subgroup 5 failed a test for special causes. Now, to look for small shifts away from the target, you want to plot the CUSUMs.

1    Open the worksheet CRANKSH.MTW.

2    Choose Stat > Control Charts > Time-weighted Charts > CUSUM.

3    Choose All observations for a chart are in one column, then enter AtoBDist.

4    In Subgroup sizes, enter 5. Click OK.

Session window output

Graph window output

Interpreting the results

The CUSUMs for subgroups 4 through 10 fall outside the upper control limit, suggesting small shifts away from the target.