Quality Advisor

Analyze for special cause variation

The key to chart interpretation is to initially ascertain the type of variation in the system—that is, whether the variation is coming from special or common causes. When the system has only common causes of variation, it is referred to as stable or in control. If, however, the system has special causes of variation, it is referred to as unstable, or out of control.

Look any of the conditions listed below, which indicate that the process is statistically unstable:

>> Any point lying outside the control limits
>> 7 or more points in a row above or below the centerline
>> 7 or more points in one direction
>> Any nonrandom pattern
>> Too close to the average
>> Too far from the average
>> Cycles
>> Trends
>> Clusters
>> Sawtooth
>> 2 of 3 points beyond 2 sigma
>> 4 of 5 points beyond 1 sigma

When you have determined whether or not there is special cause variation, declare the system stable or unstable.