"The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information"[1] is one of the most highly cited papers in psychology.
The information contained in the input can be determined by the number of binary decisions that need to be made to arrive at the selected stimulus, and the same holds for the response.
Memory span refers to the longest list of items (e.g., digits, letters, words) that a person can repeat back in the correct order on 50% of trials immediately after the presentation.
The number seven constitutes a useful heuristic, reminding us that lists that are much longer than that become significantly harder to remember and process simultaneously.
Chunking is used by the brain's short-term memory as a method for keeping groups of information accessible for easy recall.
[9] Baddeley used this finding to postulate that one component of his model of working memory, the phonological loop, is capable of holding around 2 seconds of sound.
[12] Several other factors also affect a person's measured span, and therefore it is difficult to pin down the capacity of short-term or working memory to a number of chunks.
A similar feat was informally observed by neuropsychologist Oliver Sacks and reported in his book 1985 The Man Who Mistook His Wife for a Hat.
Autism expert Daniel Tammet has suggested, however, that the children Sacks observed may have pre-counted the matches in the box.