Peterson and Peterson (1959)

Aim: To investigate the duration of short-term memory (STM).

Method: Peterson and Peterson conducted two experiments investigating the duration of STM. This summary focuses on the first experiment. Peterson and Peterson used a repeated measures design involving 24 students from Indiana University. Each participant sat beside the experimenter, who read aloud a three-letter consonant syllable, known as a trigram, such as CHJ. The experimenter then gave the participant a three-digit number, such as 506. To prevent rehearsal, the participant counted backwards in threes or fours from this number. After a predetermined interval, a red light appeared, signalling that the participant should recall the trigram. Each participant was tested at intervals of 3, 6, 9, 12, 15 and 18 seconds.

Results: Peterson and Peterson found that:

• recall became poorer as the interval between presentation and recall increased

• after 3 seconds, more than 50% of the trigrams were recalled correctly

• after 18 seconds, fewer than 10% of the trigrams were recalled correctly (see Graph 3)

Conclusion: Peterson and Peterson concluded that information remains in STM for approximately 18 seconds when rehearsal is prevented.