In the context of performance improvement and quality control, variations in a process are typically categorized as either common cause or special cause12345.
Common cause variation is the kind of variation that is part of a stable process. These are variations that are natural to a system and are quantifiable and expected1. They are predictable, ongoing, and consistent1. Major changes would typically have to be made in order to change the common cause variations1. You can identify common cause variation points on the control chart of a process measure by its random pattern of variation and its adherence to the control limits1.
Special cause variation, on the other hand, is unexpected variation in the process14. There is a specific cause that can be assigned to the variation4. These variations are unusual, unquantifiable, and are variations that have not been observed previously, so they cannot be planned for and accounted for1. These causes are typically the result of a specific change that hasoccurred in the process, with the result being a chaotic problem1. You can identify special cause variation on a control chart by their non-random patterns and out-of-control points15.
In the given scenario, the performance improvement team has been examining delays in getting admissions from the emergency room to the nursing units. After six months of collecting data, the upper control limit was 150 minutes, and the lower control limit was 60 minutes. The next month’s data shows a time of 155 minutes. This time of 155 minutes is beyond the established upper control limit of 150 minutes. Therefore, this represents a special cause variation15, as it is an unexpected variation that significantly deviates from the established control limits.
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