AAOM Handbook

There are three significant causes of excessive variation in Process output. These are; • Excessive variation in the inputs, • Excessively variable performance of the Process steps, and • Over reaction (un-necessary change – usually made by management) to normal variation in the Process. If the inputs to a Process exhibit variation then either the Process must be less efficient through trying to respond to and control the potential effects of this input variation, or the variation will flow through to the output. Removal of variation at the input is frequently the most cost effective way to reduce overall Process variation. It is obvious that if a machine or person carrying out a step in the Process produces an output that has excessive variation then this may flow through to other steps and finally to the output. To remove variation these causes must be identified and removed/reduced. People are inherently far less consistent in the performance of tasks than machines, hence much of our effort towards reducing variation in Stable Processes should be directed towards achieving standardised completion of the work people perform. However, measuring, understanding and improving the standard of people's work performance is probably the most neglected or avoided improvement opportunity. Instead, managers will consistently modify plant, specifications and procedures in response to variation in performance that had a root cause in misuse of plant, or neglect of specifications or procedures. The third of the significant causes of variation is less intuitive or obvious for people to recognise. It is that when we make a Process change in response to normal variation, that change produces an increase in variation, not a reduction. This result can be demonstrated with simple experiments or, for the brave, the Process they

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