My Reading on ASQ CQA HB Part V Part 2

Charlie Chong/ Fion Zhang
Part VD1
Common Causes
Variation that is always present or inherent in a process is called common cause variation It occurs when one
or more of the six previously mentioned factors fluctuate within the normal or expected manner and can be
improved only by changing a factor. Common causes of variation occur continually and result in controlled
variation. They ensue, for example, from the choice of supplier, quality of inputs, worker hiring and training
practices, equipment selection, machinery maintenance, and working conditions. If the process variation is
excessive, then the process must be changed. Eradicating these stable and predictable causes of variation is
the responsibility of the managers of the process. Common causes are beyond the control of workers, as was
demonstrated by Deming’s famous red bead experiment.1 In that experiment, volunteers were told to produce
only white beads from a bowl containing a mixture of white and red beads. Monitoring or criticizing worker
performance had no effect on the output. No matter what the workers did, they got red beads—sometimes
more, sometimes less, but always some—because the red beads were in the system. Deming estimated that
common causes account for 80 percent to 95 percent of workforce variation. This is not the fault of the workers,
who normally do their best even in less- than-ideal circumstances. Rather, this is the responsibility of the
managers, who work on, not in, the process. Management decides how much money and time is to be spent on
designing processes, which impacts the resources and methods that can be used. It is the design of the
process that impacts the amount of common cause variation.