Common causes are poor light, humidity, vibration, poor food on cafeteria, absence of a real quality program, poor supervision, poor instruction, procedures not suited to the requirements, poor arrangements for comfort of workers.
They are faults of the system, so they usually stay there until they are removed by management.
Common causes are the normal, expected variances that occur.
Common causes are predictable and are not considered unusual.
Many authors have studied the economic design of the control chart for a single assignable cause.
But, in practice, multiple assignable causes are more logical and realistic.
Edwards Deming, as statistician, also would like to say there were two main kind of causes, common and special.
Common or Environmental causes tend to be the 85% of the cases: are called common because affect equally all workers in a section.
As we don’t see them we don’t analyze them, so the solutions we apply often are just Band-Aids that only hide for a while the real root cause.
In the end the problem goes back and we keep wasting time and money. As Dr Edwards Deming used to say, “If you want to control your destiny: always you can understand variation everywhere”.