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The term variation refers to changes or differences in something within certain limits. All systems contain variation. Knowledge of the type of variation is needed to determine appropriate actions in each case.

  • Common causes of variation are those that are inherent in a process or system. They affect everyone working in the process and affect all outcomes. A system or process that only contains common causes of variation is often called “stable” or “statistically stable” or “predictable” within statistically-established limits. It means that improvement can only be achieved through fundamental changes to the process or system.
  • Special causes of variations are those that are not part of the process or system all the time or do not affect everyone but arise because of unusual circumstances. A system that has both common and special causes of variation is called “unstable” or “”
  • Structural variation is a sub-set of special causes of variation. It’s a repeating pattern of variation that is inherent in the process due to environmental factors. Examples: increases in emergency room visits on a Friday night; seasonality in employment rates.
  • Intended variation is an important part of effective patient-centred health care, for example, when a provider, process or system responds purposefully to a patient’s individual needs.
  • Unintended variation is due to changes introduced into health care processes that are not purposeful, planned or guided. Most improvement work is focused on reducing unintended variation.
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