of automatic, expert performance will require detailed consideration of the
knowledge acquired with practice, rather than simply the changes in process-
ing which occur.
Action Slips
Some of the theoretical notions considered so far in this chapter are relevant to
an understanding of action slips (the performance of actions that were not in-
tended). At the most general level, it seems clear that attentional failures usu-
ally underlie action slips, and this is recognised at a commonsensical level in
the notion of ‘‘absent-mindedness.’’ However, there are several different kinds
of action slips, and each one may require its own detailed explanation.
Diary Studies
One of the main ways of studying action slips is to collect numerous examples
via diary studies. Sellen and Norman (1992, p. 317) gave the following exam-
ples of action slips from a diary study: ‘‘I planned to call my sister Angela but
insteadcalledAgnes(theyaretwins).WhatIheardmyselfsaydidnotmatch
whatIwasthinking,’’and‘‘Iwantedtoturnontheradiobutwalkedpastit
and put my hand on the telephone receiver instead. I went to pick up the phone
and I couldn’t figure out why.’’
In one diary study, Reason (1979) asked 35 people to keep diaries of their
action slips over a two-week period. Over 400 action slips were reported, most
of which belonged to five major categories. Forty percent of the slips involved
storage failures, in which intentions and actions were either forgotten or recalled
incorrectly. Reason (1979, p. 74) quoted the following example of a storage
failure: ‘‘I started to pour a second kettle of boiling water into a teapot of
freshly made tea. I had no recollection of having just made it.’’
A further 20% of the errors weretest failuresin which the progress of a
planned sequence was not monitored sufficiently at crucial junctures. An illus-
trative test failure from one person’s diary went as follows (Reason, 1979,
p. 73): ‘‘I meant to get my car out, but as I passed through the back porch on
my way to the garage I stopped to put on my wellington boots and gardening
jacket as if to work in the garden.’’Subroutine failuresaccounted for a further
18% of the errors; these involved insertions, omissions, or re-orderings of the
component stages in an action sequence. Reason (1979, p. 73) gave the follow-
ing example of this type of error: ‘‘I sat down to do some work and before
starting to write I put my hand up to my face to take my glasses off, but my
fingers snapped together rather abruptly because I hadn’t been wearing them
in the first place.’’
There were relatively few examples of action slips belonging to the two
remaining categories ofdiscrimination failures (11%) andprogramme assembly
failures(5%). The former category consisted of failures to discriminate between
objects (e.g. mistaking shaving cream for toothpaste), and the latter category
consisted of inappropriate combinations of actions (e.g. Reason, 1979, p. 72): ‘‘I
unwrapped a sweet, put the paper in my mouth, and threw the sweet into the
waste bucket.’’
Attention and Performance Limitations 389