Human error

Human Error is commonly defined as a failure of a planned action to achieve a desired outcome. Performance shaping factors (PSFs) exist at individual, job, and organisational levels, and when poorly managed can increase the likelihood of an error occurring in the workplace. When errors occur in hazardous environments, there is a greater potential for things to go wrong. By understanding human error, responsible parties can plan for likely error scenarios, and implement barriers to prevent or mitigate the occurrence of potential errors.

Errors result from a variety of influences, but the underlying mental processes that lead to error are consistent, allowing for the development of a human error typology. An understanding of the different error types is critical for the development of effective error prevention and mitigation tools and strategies.  A variety of these tools and strategies must be implemented to target the full range of error types if they are to be effective.

Errors can occur in both the planning and execution stages of a task. Plans can be adequate or inadequate, and actions (behaviour) can be intentional or unintentional. If a plan is adequate, and the intentional action follows that plan, then the desired outcome will be achieved. If a plan is adequate, but an unintentional action does not follow the plan, then the desired outcome will not be achieved. Similarly, if a plan is inadequate, and an intentional action follows the plan, the desired outcome will again not be achieved. These error points are demonstrated in the figure below and explained in the example that follows.

Human error – failures in planning and execution


Image - Figure 1 - Diagram - Human error



 Example: Failure of plans and actions

Sam has finished his last task for the day and his desired outcome is to get to the accommodation module. He knows that his usual path to the accommodation module has been barricaded off, so he plans a different route to get there. From a human error perspective, there are three potential alternative scenarios that he may experience when executing his plan:

Outcome 1 - Success: The accommodation module is easily reached via his planned route (adequate plan), and he follows that route (intentional action), arriving at the accommodation module.

Outcome 2 - Error: The accommodation module is easily reached via his planned route (adequate plan), but as he starts walking, his brain switches to autopilot. He follows his usual path (unintentional action) until he reaches the barricade, and has to retrace his steps.

Outcome 3 - Error: His planned route does not lead to the accommodation module (inadequate plan), and he follows the planned route (intentional action), until he arrives at a workshop and has to find a different route to get to the accommodation module.


Each of the failure types can be further broken down into categories and subcategories, as detailed in the following section.

Human error typology

Failures of action, or unintentional actions, are classified as skill-based errors. This error type is categorised into slips of action and lapses of memory. Failures in planning are referred to as mistakes, which are categorised as rule-based mistakes and knowledge-based mistakes.

 Image - Figure 3 - Diagram - Human error


Skill-based errors

Skill-based errors tend to occur during highly routine activities, when attention is diverted from a task, either by thoughts or external factors. Generally when these errors occur, the individual has the right knowledge, skills, and experience to do the task properly. The task has probably been performed correctly many times before. Even the most skilled and experienced people are susceptible to this type of error. As tasks become more routine and less novel, they can be performed with less conscious attention – the more familiar a task, the easier it is for the mind to wander. This means that highly experienced people may be more likely to encounter this type of error than those with less experience. This also means that re-training and disciplinary action are not appropriate responses to this type of error.

A memory lapse occurs after the formation of the plan and before execution, while the plan is stored in the brain. This type of error refers to instances of forgetting to do something, losing place in a sequence, or even forgetting the overall plan. 

A slip of action is an unintentional action. This type of error occurs at the point of task execution, and includes actions performed on autopilot, skipping or reordering a step in a procedure, performing the right action on the wrong object, or performing the wrong action on the right object. Typical examples include:

  • missing a step in an isolation sequence

  • pressing the wrong button or pulling the wrong lever

  • loosening a valve when intending to tighten it

  • transposing digits when copying numbers (e.g. writing 0.31 instead of 0.13)


Slips and lapses can be minimised and mitigated through workplace design, effective fatigue management, use of checklists, independent checking of completed work, discouraging interruptions, reducing external distractions, and active supervision.



Mistakes are failures of planning, where a plan is expected to achieve the desired outcome, however due to inexperience or poor information the plan is not appropriate. People with less knowledge and experience may be more likely to experience mistakes. Mistakes are not committed ‘on purpose’; as such, disciplinary action is an inappropriate response to these types of error.  


Mistakes can be minimised and mitigated through robust competency assurance processes, good quality training, proactive supervision, and a team climate in which co-workers are comfortable observing and challenging each other. 


Mistakes can be rule-based or knowledge-based. The different types of mistakes are explained below through the use of an example from NOPSA Safety Alert 28, where a construction vessel failed to avoid a cyclone. This example demonstrates how multiple errors at various levels of an organisation can interact to lead to a hazardous event.


Image - Figure 6 - Diagram - Human error


Knowledge-based mistakes result from ‘trial and error’. In these cases, insufficient knowledge about how to perform a task results in the development of a solution that is incorrectly expected to work.


 Example - Knowledge-based mistake

A construction vessel was unable to avoid a cyclone because the operator failed to initiate preparations to evacuate in a timely manner. The risk of tropical lows rapidly developing into cyclones within the Timor Sea location was not well understood, with insufficient time allocated for evacuation tasks.


Rule-based mistakes refer to situations where the use or disregard of a particular rule or set of rules results in an undesired outcome.

Some rules that are appropriate for use in one situation will be inappropriate in another. Incorrect application of a good rule occurs when a rule has worked well on previous occasions, so it is applied to a similar situation with the incorrect expectation that it will work. 


Example - Incorrect application of a good rule

The operator of the construction vessel planned for cyclone response times based on typical North West Shelf cyclone development patterns. These time frames were then applied to a Timor Sea location. In this case, a good rule regarding evacuation time frames was incorrectly applied to a different location.


Sometimes rules are inappropriate or incorrect, and adherence leads to negative outcomes. In these cases, application of a bad rule does not deliver the desired outcome. Bad rules may be created based on incorrect knowledge (i.e. knowledge-based mistakes), or a good rule may become bad following changes that are not managed appropriately.


Example - Application of a bad rule

The two mistakes described above were incorporated into the Cyclone Response Procedure. When applied, this bad rule allowed insufficient time for evacuation and associated preparation times before the tropical low developed into a cyclone. It is fortunate that no injuries were experienced during this situation.


Failure to apply a good rule is also known as a violation. Violations are classified as human error when the intentional action does not achieve the desired outcome, or results in unanticipated adverse consequences. Violations tend to be well-intentioned, targeting desired outcomes such as task completion and simplification. Where violations involve acts of sabotage designed to cause damage, the planned action (violation) has achieved the desired outcome (damage). This type of behaviour does not constitute human error and, following investigation, should be managed through the application of appropriate disciplinary measures. There are three main types of violations pertaining to human error: routine, situational, and exceptional.

Image - Figure 10 - Human error

A routine violation is one which is commonplace and committed by most members of the workplace. For example, in a particular office building it is against the rules for personnel to use the fire escape stairwell to move between floors, but it is common practice for people to do so anyway.



 Example - Routine violation – BP Texas city refinery explosion, 2005

At BP’s Texas city refinery, procedures were outdated and ineffective in managing operational problems frequently encountered during the raffinate unit start-up procedures. As a consequence, operators believed that procedures did not have to be followed or could be altered during startup. It was routine practice during ISOM unit start-up to fill the raffinate splitter tower above the range of the level transmitter, although procedures required the tower to be filled to a 50% reading. This is just one of many routine violations that occurred on the day of the explosion


U.S. Chemical Safety and Hazard Investigation Board (2007). Investigation Report: Refinery Explosion and Fire, BP, Texas City, Texas, March 23, 2005.

A situational violation occurs, as its name suggests, in response to situational factors, including excessive time pressure, workplace design, and inadequate or inappropriate equipment. When confronted with an unexpected or inappropriate situation, personnel may believe that the normal rule is no longer safe, or that it will not achieve the desired outcome, and so they decide to violate that rule. Situational violations generally occur as a once-off, unless the situation triggering the violation is not corrected, in which case the violation may become routine over time.


 Example - Situational violation - BP Texas city refinery Explosion, 2005

On the day of the explosion, the day-shift supervisor left work at short notice to attend to a family emergency. BP procedures required the presence of an experienced supervisor during start-up activities. However, no-one was assigned to replace the day-shift supervisor, and work was not stopped until an appropriate replacement could be sourced. This example demonstrates how violations are also committed by decision-makers, not just by frontline personnel.


An exceptional violation is a fairly rare occurrence and happens in abnormal and emergency situations. This type of violation transpires when something is going wrong and personnel believe that the rules no longer apply, or that applying a rule will not correct the problem. Personnel choose to violate the rule believing that they will achieve the desired outcome.

Preventing violations requires an understanding of how motivation drives behaviour. Planned behaviour (intentional action) is driven by an individual’s attitude towards that behaviour. Further, individual decision-making is primarily influenced by the consequences the individual expects to receive as a result of their behaviour, which can influence their attitude towards that behaviour.

In most organisations, consequences associated with risk management behaviours compete against those associated with productivity behaviours. While ‘Safe Production’ is a popular phrase, risk management activities necessarily increase the amount of time required to complete a task. Productivity outcomes are generally more predictable and definitive than those associated with risk management (i.e. definitely achieving a target versus potentially avoiding an incident). So the perceived value of productivity behaviour may be greater than that of risk management behaviour.


Violations can be minimised and prevented through education about risks and consequences, training in 'why' not just 'how', the use of decentralised decision-making structures, dedicated site-based roles of procedure modification approval, allowing sufficient time for risk management activities, the use of lead indicators as targets, and active workforce involvement in the development of rules and procedures that will affect them.


Note: Violations are classified as human error only when they fail to achieve the desired outcome. Where a violation does achieve the desired outcome, and does not cause any other undesired outcomes, this is not human error. These types of violations may include violation of a bad rule, such as a procedure that, if followed correctly, would trip the plant. In such cases, a review of the rules and procedures is advisable.


 Example - Violating a bad rule - Piper Alpha muster

During the Piper Alpha disaster, personnel following the muster procedures found that they could not access the lifeboats from the accommodation block. Personnel who survived the disaster were those who chose to violate the muster rule and ‘step off’ the platform into the ocean. In this case a bad rule was violated and the desired outcome was achieved.