C10. Unintentional Errors


Errors (Slips & Lapses)


Errors are unintended or unplanned actions. These occur in routine tasks with operators who know the process well and are experienced in their work. They often involved missing a step out of a sequence or getting steps in the wrong order and frequently arise from a lapse of attention, for example operating the wrong control through a lapse of attention or accidentally selecting the wrong gear.


These are unlikely to be eliminated by training and need to be designed out. Design improvement is the most effective route for eliminating the cause of this type of human error.


For example, typical problems with controls and displays that cause this type of error include:

  • Switches which are too close and can be inadvertently switched on or off.
  • Displays which force the user to bend or stretch to read them properly.
  • Critical displays not in the operators field of view.
  • Poorly designed gauges.
  • Displays which are cluttered and / or are difficult to read.
  • Displays showing non-essential information.





Mistakes are errors of judgement. They are decisions that at the time the operator believed to be correct, but are subsequently found to be wrong.


There are 2 types of mistake:


1. Rule Based

A rule-based mistake occurs when an inappropriate action is tied to a particular event.


2. Knowledge Based

A Knowledge based error when the person must rely entirely on adapting their basic knowledge and experience to deal with a new problem.


Training is the key to avoiding mistakes. Training should be based on defined training needs and objectives and it should be evaluated to see if it has had the desired improvement in performance.