sources of variation

“Variation there will always be, between people, in output, in service, in product. What is the variation trying to tell us?” (Deming)

“Of all the issues that an Engineer has to resolve, dealing with variation is the most difficult.” (T. Davies, Ford)

Blaming people for problems beyond their control

Spending money for new equipment that is not needed

Wasting time looking for explanations of a perceived trend when nothing has changed

Taking other actions when it would have been better to do nothing

The concept of sources of variation can be used to help minimize these and other losses resulting from misinterpretation of variation.

The variety of goods or services being offered.

Structural variation in demand. Random variation. Assignable variation.

Common Cause variation is created by many factors that are commonly part of the process, and are acting totally at random and independent of each other

For instance, older machines generally exhibit a higher degree of natural variability than newer machines.

Inappropriate procedures Poor design Poor maintenance of machines Lack of clearly defined standard

operating procedures  Poor working conditions, e.g.

lighting, noise, dirt, temperature, ventilation

Special Cause variation is created by a non-random event leading to an unexpected change in the process output.

For example, the attentiveness of 50 people at a presentation is a affected by causes that are common to all of them. There are also causes that affect attentiveness that are special to individuals, such as lack of sleep, family problems, and health.

A process (or a system) that has only common causes affecting the outcomes is called stable process or said to be in state of statistical control.

A process whose outcomes are affected by both common causes and special causes is called an unstable process.

Interpretations of variationVariation that indicates good or bad performance

Variation that results from common or special causes

Focus

Aim

Basis

Methods

Outcomes of the process (product or service)Classify outcomes as acceptance or notWhat the customer wants or needsSpecifications, budgets, forecasts, numerical goals

Causes of variation in the processProvide a basis for action on the processWhat the process is actually deliveringControl charts

We could mistake the cause of variation as being special in nature, when in fact it is random and caused by the system (common cause)

We could mistake the source of variation as being systemic in nature (common causes), when in fact it is special in nature (special cause) and can and should be identified and, if possible eliminated

“Confusion between common causes and special causes leads to frustration of everyone, and leads to greater variability and to higher costs, exactly contrary to what is needed.” Denim