Capacity

• Digit span


  Jacobs (1987) measured digit span to investigate capacity. The researcher reads out four digits and the participants recalls these out loud in order. If correct, the researcher adds a digit each time until the participant can no longer recall them correctly. This indicates the individual's digit span. Jacobs found that the mean span for digits was 9.3, and letters was 7.3.
• Span of memory and chunking

  Miller (1956) observed everyday life and found that things come in sevens; notes on the musical scale, days of the week, deadly sins, etc. Miller thought that the span (capacity) of STM is about seven items, plus or minus two. He also noted that people can recall five words as easily as they can recall five letters due to chunking - grouping sets of digits or letters into units or chunks.