Short Term Memory

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Duration of short-term memory

Your memory for immediate events. The short term memory store has limited duration. 

Peterson and Peterson study:

  • Help of 24 students attending university. 
  • Recal was tested by the experimenter saying a nonsense trigram to the particpant followed by 3 digit numbers.
  • After hearing the trigram and the 3 digit number, they had to count backwards in 3s until they were told to stop and then was asked to recall the trigram and 3 digit number
  • Participants remembered 90% when there was a 3 second interval and about 2% when there was an 18 second interval.
  • This suggests that STM last about 20 seconds at most


  • They are only studying one kind of memory, memory for syllables and words.
  • They were not testing for duration. When the participants were counting backwards the trigram could have been displaced in STM.  
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Duration of long-term memory

Your memory for events that have happened in the past. The could last anywhere from 2 minutes to 100 years+. The long-term memory store has potentially unlimited duration.

Shepard (1967) tested duration of LTM. 

Procedure and findings:

  • Showed particpants 612 memorable pictures.
  • 1 hour later they were shown some of these pictures among a set of others and showed almost perfect recognition.
  • 4 months later theywere able to recognise 50% of the photographs.

Bahrick et al (1975) done another experiment which showed more meaningful pictures to the participants. Therefore the duration of the long term memories were much better. He asked them to put names to faces from their high school year book. 48 years on, people were about 70% accurate. 

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Capacity of short-term memory and long-term memory

Capacity - Measure of how much can be held in memory/

Long-term memory has potenially unlimited capacity.

Capacity of short-term memory, about 7±2, George Miller (1956). Millr also found that we can recall 5 words as well as we can recall 5 letters - we can chunk things together and can then remember more.

Chunking - Miller proposed the capacity of STM can be enhanced by grouping sets of digits or letters and making them into useful units or 'chunks'.

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Encoding of short-term memory and long-term memory

Short-term meory uses acoustic and visual enconding and long-term memory uses semantic enconding. 
Acoustic encoding involes coding information in terms of the way it sounds.
Semanting encoding incoles coding information in terms of its meaning.

Baddely (1966) tested the effects of acoustic and semantic enconding on short and long term recall.

Procedure and findings:

  • Gave particpants a lists of words which were acoustically similar or dissimliar and words which were semantically similar or dissimliar.
  • He found that particpants had diffuculty rembering words acoustically similar in STM but not in LTM
  • Whereas semnatically similar words posed little problem for short-term recall but led to muddeled long-term memories.  
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