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The differences in result obtained in measuring the same phenomenon more than once.
Concept of Variations:
The sources of Variation in a process over time can be grouped into two major classes:
1.Common cause variation
2.Special cause variation
1.Common Cause Variations:
The common cause variation (endogenous cause variation or systemic cause variation) is always present and is part of the variation inherent in all processes. They are endogenous to a system and not disturbance (they are the system) and can be removed or eliminated only by making basic changes in the system. The origins can usually be traced to an element of the system that the organization can correct or improve. Some sources include design of a system, choice of equipment and preventive maintenance of equipment.
2. Special Cause Variations:
(Assignable cause variation, attributable cause variation, exogenous cause variation or extra-systemic cause variation) are not inherently present in the system itself. They arise from causes that are not part of the system as designed. They are intermittent and indicate that a process is unstable. When they are operating within a system, the output of that system or process is impossible to predict or control. Ideally once detected, they should be eliminated by an organization, leaving the process with only common cause variations. They tend to cluster by person, place or time. Occasionally they can result in better outcomes, where they should be incorporated into the conventional system so that better outcome will recur.
Neither type of variation is good or bad in itself.
Management Strategy for the Types of Variations:
When a process exhibits special cause variation, the change effort should focus on investigating the origin of the special cause, and not on changing the process. For example, if the mortality rate for 10 months averaged 4 percent, with monthly averages varying slightly above or below 4 percent, this would suggest (''common cause variation'').
Then if the mortality rate in one month jumped to a 10 percent rate, the surgeons should not respond by changing their operative procedures (the process),but should first investigate the reason for the big jump in mortality for that month ( the ''special cause variation'').
When a process exhibits only common cause variation but is functioning at an unacceptable level, management strategy should focus on reducing the amount of variation and moving the process average in the desired direction. This strategy involves implementing successive changes in the process and evaluating the effect of each change. This strategy is often referred to as the '' Plan-Do-Study-Act,'' (PDSA, Cycle):
1.Plan a change. This means modifying the current process in some way, or perhaps redesigning it completely.
2.Do it in a trial or on a small test group.
3.Study the results. Ask weather the new process has a level of performance and/or random variation that is superior to that displayed by the old process.
4.Act. Either implement the tested alternatives, modify it and test again, or discard it.
Diagnostic Tools for Variations:
Both are diagnostic tools to assist managers of a process in determining the type of variation.
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