RE: Special cause / Common cause

["Mack, Wayne" <wayne.mack@pec.com>]

 Let me pass along a clarification on "Special" and "Common" causes I received.
Another term for a Special cause is an Assignable cause.  

When we say assignable or special cause, we mean that for a particular data
point we can identify the reason that it differs from the others.  In the
example below, a broken pin would be an assignable/special cause problem for
the manufacturer.  The variation in product received by the customer may or
may not be common cause.  If the use of the product received caused
significant issues, then it would be assignable cause variation, but it
would probably be attributed to specific lots received from the
manufacturer, not to a broken pin.

Special or assignable causes are identified more by the magnitude of the
variation they cause rather than the uniqueness or repetition rate.

Wayne Mack