In this section, we consider action slips (the performance of actions that were not intended). It is clear that attentional failures are usually involved in action slips, and this is recognised at a commonsense level in the notion of "absent-mindedness". However, there are several kinds of action slips, and each one may require its own detailed explanation.
One way of studying action slip is to via diary studies. Sellen and Norman (1992, p. 317) gave the following example of an action slip from a diary study: "I wanted to turn on the radio but walked past it and put my hand on the telephone receiver instead. I went to pick up the phone and I couldn't figure out why."
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 were test failures in which the progress of a planned sequence was not monitored sufficiently at crucial junctures. Here is an example of a test failure (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 failures accounted 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 following 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 only a few action slips in the two remaining categories of discrimination failures (11%) and programme 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."
It would be unwise to attach much significance to the percentages of the various kinds of action slips. The figures are based on those action slips that were detected, and we simply do not know how many cases of each kind of slips went undetected. The number of occurrences of any particular kind of action slip is meaningful only when we know the number of occasions on which that kind of slip might have occurred but did not. Thus, the small number of discrimination failures may reflect either good discrimination or a relative lack of situations requiring anything approaching a fine discrimination.
Another issue is that two action slips may seem similar, and so be categorised together, even though the underlying mechanisms are different. For example, Grudin (1983) conducted videotape analyses of substitution errors in typing involving striking the key adjacent to the intended key. Some of these errors involved the correct finger moving in the wrong direction, whereas others involved an incorrect key being pressed by the finger that normally strikes it. According to Grudin, the former kind of error is due to faulty execution of an action, whereas the latter is due to faulty assignment of the finger. We would need more information than is generally available in most diary studies to identify such subtle differences in underlying processes.
Several techniques have been used to produce action slips in laboratory conditions. What is often done is to provide a misleading context that increases the activation of an incorrect response. Reason (1992) discussed a study of the "oak-yolk" effect illustraing this approach. Some participants were asked to respond as rapidly as possible to a series of questions (the most frequent answers are given):
What do we call the tree that grows from acorns?
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