Special Causes of Variation

Special Causes of Variation lead to an uncontrollable variation whose nature is unpredictable, and which arises from outside influences not inherent in the process itself.

Some examples of Special Causes are:

• a change in materials (e.g. new supplier, different paper)
• a breakdown in the machine or process (e.g. tool wear, postal strike)
• a different operator (e.g. new trainee)
• environmental changes (e.g. unusually bad weather)

Special Causes must always be investigated as soon as possible after it becomes apparent their presence is having an adverse effect on the process and its outputs.

When the reason for the adverse special cause has been identified it should, as far as possible, be eliminated from the process in such a way that its reappearance is extremely unlikely. Such actions might be the tightening up of sickness procedures, or introducing regular maintenance to a machine.

Alternatively, there are some occasions when a special cause could have a beneficial effect on the process (e.g. a change in procedure). Again, the reason for this improvement should be investigated and, if possible, made a permanent partof the process.