4 of 5 beyond 1 sigma

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 four out of five consecutive points lie beyond the 1-sigma limit on one side of the average, the system is declared unstable.

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