Thursday, January 27

Jacobs (1887)

An early study of STM capacity

Even people who haven't studied Psychology can probably tell you about the 'magical number' 7 (plus or minus two) - the supposed maximum number of items we can hold in our short-term memories.

The best known study is Miller (1956), who tried to investigate the reason for the limit, finding that it was not limited to information load - the rationale being that a short word contains a lot more information than a binary digit (0 or 1) but yet the limit is still in the range of 5-9 items.

However the concept was known long before Miller, so it is not strictly accurate to attribute it to him.  Ebbinghaus investigated it, and in the UK, Jacobs studied what he called 'prehension'.  Participants were presented sequences of numbers, and asked to repeat them in the correct order. The sequences are initially short, and gradually increase in length one digit at a time. A person's digit span is the point at which they can recall sequences of a certain length (e.g. seven items) correctly 50% of the time. Jacobs found a digit span of 9.3 on average - when letters were used, the average was 7.3 items. Age differences were also found, with digit span increasing through childhood.

Miller was the first to talk about 'chunks' of information, but even that concept has largely been superceded by the idea of a time-limit (how many items can be pronounced in just under two seconds), leading Schweickert & Boruff (1986) to wittily question, magic number or magic spell?

Jacobs, J. (1887). Experiments in prehension. Mind, 12, 75-79.

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