Errors: An Inherent Part of Human-System Performance - Foundations for Designing User-Centered Systems - page 288

Table 10.1 GEMS

Planning Knowledge-based mistakes

Rule-based mistakes

taxonomy

Storage Skill-based lapses

Execution Skill-based slips

• Errors of commission: actions that are performed inadequately, out of sequence,
or at the wrong time (too early/late); or are qualitatively incorrect (too much/too
little/wrong direction).
• Extraneous actions: actions that would not normally be expected at that par-
ticular time and place.

In THERP the probabilities of the different errors occurring are conditioned by
performance shaping factors (PSFs). The PSFs are intended to take some account
of the context in which the error occurs, to improve the accuracy of the individual
error probability estimates. They are usually divided into external PSFs (such as
situational characteristics), stressor PSFs (such as psychological stressors), and
internal PSFs (such as organismic factors like previous training).

10.3.2 Generic Error Modeling System

Rasmussen's ( 1976 ) SRK model of behavior (see Chap. 5 ) has been used as the
basis for several taxonomies that describe human performance in the operation of
complex control systems, including Reason's ( 1990 ) Generic Error Modelling
System (GEMS). Reason distinguishes between three types of errors: slips, lapses,
and mistakes (see also Norman 1981 ). Slips (execution failures) occur when the
person has the right intention but performs the wrong action. Lapses (memory
storage failures) occur between the formulation of an intention and the execution
of some action. Mistakes (intention failures) occur when the person initiates the
wrong plan of action for the task at hand. Within the GEMS taxonomy, errors can
be associated with planning, with storage, or with execution of actions (see
Table 10.1 ).
Skill-based errors (slips and lapses) are normally attributable to monitoring
failures. In particular, they are often linked to a lack of attention, whether delib-
erate or unintentional.
The rule- and knowledge-based errors (mistakes) are usually associated with
problem solving activities. At the rule-based level, mistakes can arise when good
rules are misapplied, or bad rules are applied. At the knowledge-based level,
mistakes usually arise due to a lack of expertise, because people do not have all the
knowledge required to perform the task at hand. Figure 10.2 shows the relation-
ship between the different types of errors in GEMS and a typical stage model of
human information processing.