Short-term memory is an activated memory that retains some items for a short period, such as the 7 digits of a phone number when you dial it, before storing the information or losing it.
In the time between searching for a phone number and dialing it, your memory of the group of numbers will disappear unless you make an effort to keep it in consciousness.
In 1959, Lloyd Peterson and Margaret Peterson (cited in Myers, 2006) wanted to know how quickly short-term memory disappears. To find out, they asked people to remember groups of three consonants (for example, HTW). To avoid memorizing the letters, they asked them to count starting from the number 100, 3 backwards out loud. After three seconds people remembered the letters only half of the time, after 12 seconds they hardly remembered them at all. This suggested to them that without active processing, the duration of short-term memories is limited.
Short-term memory is limited by duration, but also by capacity
Short-term memory is limited by duration, but also by capacity. This memory generally stores only about 7 items of information, or as George Miller calls it: “The magical number 7, give or take two.”
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Our short-term memory is a little easier for random digits than for random letters, which sometimes sound similar. However, eliminating repetition reduces memory of letters or numbers to four items. The basic principle that emerges is that at any given moment, we can consciously process only a limited amount of information.
Fountain: Myers (2006), Psychology 7th edition, Editorial Médica Panamericana: Madrid