Peterson and Peterson (1959)

AIM: To examine the duration of the short-term memory store.

METHOD: Peterson and Peterson conducted two experiments. As both experiments focused on the duration of the STM store, this summary will focus on the first of the two experiments.

Peterson and Peterson used a repeated measures design where 24 students from Indiana University took part. Each participant was seated next to the experimenter. The experimenter would spell out a three-letter consonant syllable (e.g. CHJ) and then say a three-digit number (e.g. 506). The three-letter consonant syllables are often referred to as trigrams. The participant would then count backwards in threes or fours from this number. At the end of a pre-set interval, a red light would activate. The light was a signal for the participant to recall the trigram. Each participant was tested eight times, at the following time intervals: 3, 6, 9, 12, 15 and 18 seconds.

RESULTS: Peterson and Peterson found that the longer the interval between the presentation and recall, the poorer the recall. At 3 seconds, over 50% of the trigrams were correctly recalled, whereas, at 19 seconds, less than 10% were correctly recalled (see Graph 3).

CONCLUSION:Peterson and Peterson concluded that the STM store has a capacity of approximately 18 seconds.