Error graphic

An Anatomy of Error

“We put people in boring environments with nothing to do for hours on end, until suddenly they must respond quickly and accurately. Or we subject them to complex, high workload environments, where they are continually interrupted while having to do multiple tasks simultaneously. Then we wonder why there is failure”

This is just one of the many standout quotes from Don Norman’s ‘The Design of Everyday Things‘.

The chapter on errors (yes, an entire chapter) is characteristic of the rest of the book – take behaviour, break apart and ask why it is this way.

Norman categorises two types of errors:


Slips occur when someone intends to do something but does something instead by accident.
Norman further categorises slips into two sub-classes:

  • action-based
    • wrong action is performed on an object
      “I pressed the wrong button to change channels on the TV remote”
  • memory-lapse
    • “I forgot to turn the TV off when I left for work”


Mistakes are errors from first principles where a goal or plan is flawed so it’s inevitable that it’s not going to turn out as expected despite all actions along the way being executed properly.
Here, Norman breaks this down into three sub-classes:

  • rule-based
    • a situation is correctly diagnosed but the wrong course of action is taken resulting in an error
  • knowledge-based
    • a situation, owing to incomplete or incorrect information, has been misdiagnosed resulting in an error
  • memory-lapse
    • somebody has just forgotten to do something at some stage resulting in an error

Being mindful of these potential pitfalls:

…lets us deal with error by embracing it, by seeking to understand the causes and ensuring that they do not happen again. We need to assist rather than punish or scold.

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