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Assignable Causes and Actions

Electronic Statistics Textbook Dictionary

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Assignable Causes and Actions
In the context of monitoring quality characteristics you have to distinguish between two different types of variability: Common cause variation describes random variability that is inherent in the process and affects all individual values. Ideally, when your process is in-control, only common cause variation will be present. In a quality control chart, it will show up as a random fluctuation of the individual samples around the center line with all samples falling between the upper and lower control limit and no non-random patterns (runs) of adjacent samples. Special cause or assignable cause variation is due to specific circumstances that can be accounted for. It will usually show up in the QC chart as outlier samples (i.e., exceeding the lower or upper control limit) or as a systematic pattern (run) of adjacent samples. It will also affect the calculation of the chart specifications (center line and control limits).
With some software programs, if you investigate the out-of-control conditions and you find an explanation for them, you can assign descriptive labels to those out-of-control samples and explain the causes (e.g., valve defect) and actions that have been taken (e.g. valve fixed). Having causes and actions displayed in the chart will document that the center line and the control limits of the chart are affected by special cause variation in the process.