Peterson and Peterson (1959) - decay in STM

When we talk about short-term memory (STM) in everyday life, we often think of a store that lasts for hours or days - the 'short term' nature of revising for exams, for example.

However, the term is used in psychology to refer to a much more immediate store, used to maintain information for a few seconds while it is needed.  It is responsible for the familiar scenario where there is information in your head - a phone number, or a something you decide to do - and moments later it can have disappeared.  Who hasn't walked through to another room, only to forget why they are there?

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Once it had been established that STM has a capacity which is limited to around seven items (Miller, 1956), the other major factor to test was its duration - how long information can be retained before it is forgotten through decay.

The Experiment

Peterson and Peterson (1959) and Brown (1958) both published studies using a very similar technique to test the duration of STM, and it therefore became known as the Brown-Peterson Technique.

A difficulty with studying how quickly information decays from STM is that we tend to try to maintain the information there by repeating it to ourselves, in a process known as maintenance rehearsal.  A key aspect of the Brown-Peterson technique is that rehearsal is prevented, to see in order to see how quickly information decays if a person is not actively trying to remember it.  This was done by showing people a trigram of letters or words, and then using a distractor task where people counted backwards in threes from a large number (e.g. 498, 495, 492...).

Image by DrBacchus.

Findings

Peterson and Peterson (1959) found that people's STM duration was suprisingly short.  After 3 seconds, recall stood at 90%, but after just 18 seconds, it had dropped to just 2%.

Evaluation

This was a useful experimental demonstration of the limits of STM.  However, a major weakness with the design is that while it aims to test for decay, the results could be explained by the trigrams being displaced by numbers from the distraction task. Reitman (1974) used tones instead of counting, and clearly demonstrated the major role of displacement in STM forgetting.  Also, the study used a sample of just 24 students.

References

Brown, J. (1958). Some tests of the decay theory of immediate memory. Quarterly Journal of Experimental Psychology, 10, 12-21.
Miller, G.A. (1956). The magical number seven, plus or minus two: some limits on our capacity for processing information. Psychological Review, 63(2), 81–97.
Peterson, L. R. & Peterson, M. J. (1959). Shortterm retention of individual verbal items. Journal of Experimental Psychology, 58, 193-198.