capacity, duration and coding of the STM & LTM

• Cognitive psychology


  Interested in trying to explain how memory works
• Compute metaphor


  We try to understand the mind by comparing it to a computer
• Short-term memory (STM)


  • Stores the information we are currently aware of
  • Temporary store where small amounts of information can be kept for brief periods
• Processing new information to store from sensory input


  1. Take in information from the senses and transform it into a memory trace

  2. Information is encoded (e.g. acoustically if learned through sound)
• If we do not rehearse information


  It is forgotten
• Capacity of STM


  Limited and can remember 5-9 items on average
• Duration of STM


  18-30 seconds
• Jacobs' digit span test (1887)


  1. Researcher gives a number of digits and participants have to recall them all in order

  2. Researcher increases the amount by 1 digit and participant has to recall again until they recall the correct order
• Jacobs found that on average participants remembered 9.3 digits and 7.3 letters
• Miller (1957) replicated Jacobs' study and found people can recall 7 (+ or -2) items from STM
• Chunking


  We can group information together to increase the capacity of STM
• STM


  • Limited capacity - only about 7 items can be stored at a time
  • Limited duration - storage is very fragile and information can be lost with distraction or passing of time
• Peterson & Peterson (1959) study

  1. 24 participants recalling trigrams after 3, 6, 9, 12, 15 or 18 seconds

  2. Participants counted backwards in 3s or 4s to prevent rehearsal
• The longer the time between trigrams being presented and recall, the less that was remembered
• Strengths of Peterson & Peterson study

  • Supports the multi-store model
  • High internal validity - standardised procedure with high control
• Limitations of Peterson & Peterson study

  • Sample bias - only 24 students used
  • Lacks ecological validity - artificial environment, not representative of everyday situation
• Baddeley (1966) study


  1. 72 participants placed in conditions with acoustically similar/different or semantically similar/different words

  2. Participants had to recall the words
• In STM, there is better recall of acoustically different than acoustically similar words
• Strengths of Baddeley study

  • Standardised and easily replicated, high reliability
• Limitations of Baddeley study

  • Sample bias - 72 participants not representative
  • Low ecological validity - lab study, not generalisable to everyday life
• Long-term memory (LTM)


  • Permanent memory store
  • Information in STM can be transferred to LTM if attended to long enough
  • Coding in LTM is semantic - information holds meaning
  • Capacity is potentially unlimited and can hold information for years
  • Duration is potentially a lifetime
• Bahrick (1975) study


  1. 400 participants aged 17-74 asked to remember names and recognise photos of ex-classmates

  2. Tested at different time intervals after graduation
• Participants were 90% accurate in identifying names and faces within 15 years of graduation, but this dropped to 80% verbally and 70% visually after 48 years
• Participants were better at photo recognition than free recall, with 60% accuracy after 15 years and 30% after 48 years
• Strengths of Bahrick study

  • High external validity - used meaningful stimuli
• Limitations of Bahrick study


  • Did not control for confounding variables as memories may have been rehearsed over the years
• human memory is the process by which we retain information about events that have happened in the past
• coding is the format in which the information is stored in the memory
• capacity is the amount of information held in a memory store
• duration is the length of time information can be held in the memory