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Short term memory stores information for short periods of time, about 15-30 seconds. George Miller described its limitation in capacity in his well-known and widely-cited 1956 paper “The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information.” Essentially, 5-9 measured units (including “chunks”) of information can be processed/stored in short-term memory at a time. This theory has been called into question in recent years, with some suggesting that working memory capacity is likely less than seven "units" and drawing more attention to the factors that can account for differences in capacity, such as levels of cognitive load or expertise (and therefore ability to “chunk” information).

Miller, G. (1956). The magical number seven, plus or minus two: some limits on our capacity for processing information. The psychological review, 63, 81-97.

Farrington, J. (2011). Seven plus or minus two. https://doi.org/10.1002/piq.20099

Shiffrin, R.M. & Nosofsky, R. M. (1994). Seven plus or minus two: a commentary on capacity limitations. Psychological Review, 101, 357-361.