Definitions of Error
Basic Tenets of Human Error
Human Factors Engineering
Human Performance
Types of Errors
Systems to Reduce Errors
Stroop Test
Swiss Cheese Model
Toxic Cascades
Lessons from Other Industries
Basic Safety Principles

Types of Errors

Reason classified errors based on Rasmussen’s 3 levels of performance

  • Skill-based errors – slips and lapses – when the action made is not what was intended
  • Rule-based mistakes – actions that match intentions but do not achieve their intended outcome due to incorrect application of a rule or inadequacy of the plan.
  • Knowledge-based mistakes – actions which are intended but do not achieve the intended outcome due to knowledge deficits.

Click on each of the images below to see examples.

Also used occasionally is the term “strong but wrong.” This refers to erroneous behavior that is in keeping with past practice rather than current circumstances. This type of error is very difficult to overcome, as people who have positive experience with a particular behavior can be very confident in their actions under slightly different circumstances, even when those actions are wrong.

Check your understanding:
Classify each of the following errors by reason’s levels.


Taking the turn to go to work on your day off, when you intended to go elsewhere

A gardener plants an azalea shrub (needs acid soil) at his seaside cottage (unsuitable saltwater condition)

An apprentice furniture designer cuts the wood panel to the wrong width by failing to account for the thickness of the saw blade

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