Human error is defined as something “not intended by the actor; not desired by a set of rules or an external observer; or that led the task or system outside its acceptable limits.” There are many ways of analysing errors to identify those that are likely to occur and those that are likely to cause problems. One such model – from Rasmussen – describes a human behaviour model that categorises behaviours as knowledge-based, rule-based and skill-based.

Knowledge-based behaviour

This occurs when someone does a task in a very conscious, mindful way. For example, when someone is already familiar with many of the basic skills associated with a task and now must do something more advanced. To do so, they may draw on many different experiences gained from previous learning to explore the right solution.
Mistakes that arise from knowledge-based working happen when the rules learnt and usually used for similar scenarios no longer appear helpful. For example, when a patient is complaining of a side effect that is not usually associated with their medicines, the side effect may or may not be related to their medicines. If appropriate deductions are not made by the healthcare professional the wrong recommendation could be made.

Rule-based behaviour

The rule-based mode occurs when someone follows simple rules about how to proceed with their tasks. These are most commonly thought of as ‘IF-THEN’ rules.

Mistakes can occur in rulebased working when situations are different to what you assume. For example, you might be assuming you are out of stock for a certain medicine, e.g. lansoprazole, because it is not on the shelf. Here the rule is IF I am looking for lansoprazole and it is not on the top shelf, THEN we must be out of stock. This rule could be wrong if someone has rearranged the shelves or it has been put away in the wrong place.

Skill-based behaviour 

The skill-based mode occurs when someone does a familiar task in a way that has become automatic and they are confident to do this.

Skill-based working can lead to slips and lapses:

• Slips occur when someone is ‘running on automatic’ and they do the wrong thing, despite having the best intention to do the right thing. For example, splitting a full pack of 28 tablets with the intention to supply 10, but putting the 10 back in the original pack and 18 in the labelled box instead of the other way around
• Lapses occur where someone forgets to do something, or even forgets that they have done something and ends up repeating it. For example, being distracted when dispensing a prescription and forgetting to provide a patient information leaflet.