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Study Notes

Capacity of Short-term Memory

AS, A-Level
AQA, Edexcel, OCR, IB

Last updated 22 Mar 2021

Miller (1956) published a famous article entitled ‘The Magical Number Seven, Plus or Minus Two’ in which he reviewed existing research into short-term memory. He said that we can hold seven 'items' in short-term memory, plus or minus two. Miller believed that our short-term memory stores ‘chunks’ of information rather than individual numbers or letters.

This can explain why we are able recall items like mobile phone numbers, which contain more than 7 digits. When we try to remember a phone number, which has 11 digits, we chunk the information into groups, for example: 0767…819…45…34, so we only need to remember four chunks of information and not 11 individual digits.


Miller’s (1956) theory is supported by psychological research. For example, Jacobs (1887) conducted an experiment using a digit span test, to examine the capacity of short-term memory for numbers and letters. Jacobs used a sample of 443 female students (aged from 8-19) from the North London Collegiate School. Participants had to repeat back a string of numbers or letters in the same order and the number of digits/letters was gradually increased, until the participants could no longer recall the sequence. Jacobs found that the student had an average span of 7.3 letters and 9.3 words, which supports Miller’s notion of 7+/-2.

Although Miller’s (1956) theory is supported by psychological research, he did not specify how large each ‘chunk’ of information could be and therefore we are unable to conclude the exact capacity of short-term memory. Consequently, further research is required to determine the each size of information ‘chunks’ to understand the exact capacity of short-term memory.

Finally, Miller’s (1956) research into short-term memory did not take into account other factors that affect capacity. For example, age could also affect short-term memory and Jacobs (1887) research acknowledged that short-term memory gradually improved with age.

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