Quality Advisor

A free online reference for statistical process control, process capability analysis, measurement systems analysis, and control chart interpretation, and other quality metrics.

Clusters

Note: Use this test for control sparingly. There is a tendency to overcontrol the system when using this test. Use it only when there is some doubt about the system’s stability.

When clustering is occurring, data appears in groups, even though there is no group of seven points in a row above or below the average. This pattern suggests that the system is “jumping” from one setting to another.

When trying to improve this process, questions should be asked about the transition periods between the clusters. What is causing the system to “jump”?

See also:
>> Any nonrandom pattern
>> Too close to the average
>> Too far from the average
>> Cycles
>> Trends
>> Sawtooth
>> 2 of 3 points beyond 2 sigma
>> 4 of 5 points beyond 1 sigma