Capacity of Memory

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Capacity: This is a measure of how much can be held in memory. It is measured in terms of bits of information such as number of digits. STM has a very limited capacity (less than 7 'chunks' of information) whereas LTM has potentially unlimited capacity.

Capacity of STM

Key Study: Miller


  • Miller made observations that everyday things come in 7s, for example seven notes on a musical scale or seven days in a week.
  • Miller also reviewed several studies that have investigated the span of STM e.g where participants counted dots flashed on the screen or were tested on recall of words.


  • Participants were reasonably accurate when 7 dots flashed on screen very inaccurate when 15 dots flashed on screen. This suggests that the span (or capacity) of STM is about 7 (+/- 2).
  • Miller also observed that people can recall five words as well as they can recall five letters, they do this by chunking - grouping sets of digits or letters into meaningful units (chunks).


- One problem is that Miller may have overestimated the capacity of STM. Cowan reviewed research on this topic, and found that capacoty of STM is more likely to be about four chunks rather than seven chunks. Cowan's finding was further supported by Vogel et al who found 4 items was about the limit for visual items. This means that the lower end of Miller's range is more appropriate.

- There are real world applications to the UK postcode system. Baddeley found that numbers were best remembered if placed between the city code and some random letters. Phone numbers and car licenses also use chunks. This shows that research on chunking has led to useful application.

Key Study: Jacobs


Jacobs tested digit span , each digit had to listen to four digits and recall these in the correct order, if they were correct they progressed on


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