Professor Reason highlights the notion of “intention” when considering the nature of error, asking the questions:
- Were the actions directed by some prior intention?
- Did the actions proceed as planned?
- Did they achieve their desired end?
Professor Reason suggests an error classification based upon the answers to these questions as shown in the below figure.
The most well-known of these are slips, lapses and mistakes.
Slips can be thought of as actions not carried out as intended or planned, e.g. “finger trouble” when dialling in a frequency or “Freudian slips” when saying something.
Lapses are missed actions and omissions, i.e. when somebody has failed to do something due to lapses of memory and/or attention or because they have forgotten something, e.g. forgetting to lower the undercarriage on landing.
Mistakes are a specific type of error brought about by a faulty plan/intention, i.e. somebody did something believing it to be correct when it was, in fact, wrong, e.g. switching off the wrong engine.
Slips typically occur at the task execution stage, lapses at the storage (memory) stage and mistakes at the planning stage.
Violations sometimes appear to be human errors, but they differ from slips, lapses and mistakes because they are deliberate illegal actions, i.e. somebody did something knowing it to be against the rules (e.g. deliberately failing to follow proper procedures).
A pilot may consider that a violation is well-intentioned, e.g. electing not to climb in response to a TCAS RA, if he is certain that the other aircraft has already initiated avoiding action. There is great debate about whether flight crew should follow SOPs slavishly, or should elect to diverge from SOPs from time to time. Whatever the case, and however well-intentioned, this would still technically constitute a ‘violation’ rather than an error.
Image: Error types based on intention. Source: Reason, 1990.