A free online reference for statistical process control, process capability analysis, measurement systems analysis, control chart interpretation, and other quality metrics.
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.
Notice that the plot of averages drifts upward on this example, even though there is no group of seven points in a row going up. This pattern indicates a gradual change over time in the characteristic being measured.
>> Any nonrandom pattern
>> Too close to the average
>> Too far from the average
>> 2 of 3 points beyond 2 sigma
>> 4 of 5 points beyond 1 sigma