Variability Reduction

Variability Reduction is a multi-part strategy to reduce product variation and make a product more robust or fit to use, e.g., meet its performance requirements regardless of variation.

Broadly speaking, there are three sources of variation: manufacturing variation, environmental/deterioration variation and usage variation. To the extent we are aware of these potential sources of variation, we can take steps to design a product to counter these sources of variation.


This is the concept of robust design. It is achieved primarily by the use of design of experiments to determine which factors (product and process parameters) are most sensitive to variation or noise and which factor level settings (parameter values) minimize the variability in the desired performance parameter.

Design of Experiments (DOE) can be used to counter all three sources of variation. In addition, other steps can be taken to counter manufacturing variation and usage variation. First, understanding the statistical capability of a process (process capability) can help to either design within the capability of that process or determine when an improved capability is required (e.g., new equipment, outsourcing, etc.). Second, by using SPC, special cause variation can be identified and attacked. Third, over the longer term, process variability from common causes can be systematically reduced through process optimization, operator training, preventative maintenance, tool monitoring, standardization of machine settings, climate control, power conditioning, etc. Fourth, variability in usage can be countered by mistake-proofing (see design for manufacturability guidelines), warning labels, easy-to-understand operating manuals and controls, etc.


Variability reduction involves understanding customer needs and developing a product and process design that balances these needs with process capabilities and potential sources of variation. Thus variability reduction is broader than SPC and DOE individually and more proactive than SPC. The following diagram represents an overall framework for variability reduction or quality engineering.


One source for further information is a book titled OPTIMIZATION AND VARIATION REDUCTION IN QUALITY by Wayne Taylor, 1991, McGraw Hill, ISBN 0-07-063255-3.