Duration of Short-term Memory Experiment – Peterson and Peterson

When a person tries to memorize words or numbers, they rehearse the item to remember it for a longer time. In this experiment, rehearsal was prevented leading to decay in memory.


To investigate the duration of short-term memory.

Lloyd and Margaret Peterson aimed to test the hypothesis that information which is not rehearsed is lost quickly from short-term memory.


A lab experiment was conducted in which 24 participants (psychology students) had to recall trigrams (meaningless three-consonant syllables), such as TGH, CLS.

The trigrams were presented one at a time and had to be recalled after intervals of 3, 6, 9, 12, 15 or 18 seconds respectively for each trial.

No two successive trigrams contained any of the same letters.

After hearing a trigram, participants were asked to count backward in threes or fours from a specified random number until they saw a red light appear (then they recalled the trigram). This is known as the brown peterson technique, and the purpose was to prevent rehearsal.

The independent variable was the time interval between hearing the experimenter say the trigram and the participant recalling the trigram (after seeing a red light), e.g. 3, 6, 9, 12, 15 or 18 seconds.

The dependent variable was the number of trigrams correctly recalled by the participants after every trial. The were six trials in total.


Their results showed that the longer each student had to count backwards, the less well they were able to recall the trigram accurately.

  • After 3 seconds 80% of the trigrams were recalled correctly.
  • After 6 seconds this fell to 50%.
  • After 18 seconds less than 10% of the trigrams were recalled correctly.

peterson and peterson


Short-term memory has a limited duration (of about 18 seconds) when rehearsal is prevented. It is thought that this information is lost from short-term memory from trace decay.

The results of the study also show the short-term memory is different from long-term memory in terms of duration. Thus supporting the multi-store model of memory .

If a person is not able to rehearse information, it will not transfer to their long-term memory store.

Critical Evaluation

In this study, the researchers used fixed timings for participants to count backwards from. They also eliminated noise and other factors that could have had an influence on memory.

The research can therefore be said to have good control, using standardised procedures to make sure all participants experienced the same process. This means that the study is scientific because it can be replicated and the reliability of the findings can be checked to make sure they were not a one-off result.

However, the experimental method lacked mundane realism and external validity as they used very artificial stimuli (i.e., people do not try to recall trigrams in real life).

Also, this study only considered short-term memory duration for one type of stimuli. They did not provide information about other types of stimuli such as pictures and melodies.

One important practical application of this study is that it demonstrates how interference in the form of verbal distractions can affect our ability to retain information.

This implies that when we are revising for an exam or trying to memorize a shopping list before we pop to the shop, we should take care to avoid distractions.


Peterson, L.R., & Peterson, M.J. (1959). Short-term retention of individual verbal items. Journal of Experimental Psychology, 58, 193-198

Olivia Guy-Evans

BSc (Hons), Psychology, MSc, Psychology of Education

Associate Editor for Simply Psychology

Olivia Guy-Evans is a writer and associate editor for Simply Psychology. She has previously worked in healthcare and educational sectors.

Saul Mcleod, PhD

Educator, Researcher

BSc (Hons) Psychology, MRes, PhD, University of Manchester

Saul Mcleod, Ph.D., is a qualified psychology teacher with over 18 years experience of working in further and higher education.