Theory of Variation (continued)
Example: School bus pick up times will vary because of traffic and traffic signal stops, but pick up times will be predicable within certain limits. These causes of variation are common causes.
If the bus has an accident, runs out of fuel, or a substitute driver is driving, the bus may be later than usual, outside of the predictable limits. These would be special causes of the variation.
TQM provides tools that enable one to identify special causes. (Measurement/Normal Curve)