Quality Advisor

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

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Cycles

When cycles are occurring, the data rises and falls in a rhythmic pattern. The pattern is definitely not random. This could be caused by some regular, periodic change in the system.

A positive aspect of cycles is that they tend to indicate that there is one major cause of variation, which will typically be changing in a similar cyclic fashion. If the cause of the cycle can be established and reduced, this should result in a major improvement to the process.

Cycles - chart example

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