The popular theory that human error, such as making the wrong diagnosis, operating on the wrong body part or administering the wrong medication, in itself causes harm to patients may not always be completely true. According to system safety theory and the "Swiss Cheese" theory of healthcare, at least two things have to go wrong for harm to occur. Usually, the primary cause is a poorly designed care system that allows human errors to happen. Each weakness in the system is called a "hazard." A

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... an error is a trigger event that finally results in the harm. Therefore, human error is a symptom of a poorly designed system, not necessarily the primary cause of harm. Using the analogy of a gun, the loaded gun is a hazard, while pulling the trigger can result in harm. If the gun is not loaded, the trigger (human error) is not an issue.