Capacity of STM

What is the digit span test?

• Developed by Jacobs (1887)

• Researcher gives a number of digits and participant has to recall them all in order.

• Researcher increases the amount by 1 digit and the participant has to recall again until they cannot recall the correct order

• This determines their digit span

What did Jacobs (1887) find?

Mean span for digits across all participants was 9.3 items. The mean span for letters was 7.3.

What is the capacity of STM?

7 items, plus or minus 2.

What is capacity?

The amount of information that can be held in a memory store.

What is chunking?

Grouping sets of digits or letters into units or chunks.

What did Miller (1956) find?

People can recall five words as easily as they can recall 5 letters using chunking.

One strength of Jacob's (1887) digit span test

It has been replicated. The study is a very old one and early research in psychology often lacked adequate controls. For example, some participant's digit spans might have been underestimated because they were distracted during testing. Despite this, Jacobs’ findings have been confirmed by other, better controlled studies since. This suggests that Jacobs’ study is a valid test of digit span in STM.

One limitation of Jacobs’ study (1887).

It was a lab experiment so it lacks ecological validity meaning that the tests carried out were unrepresentative to real life situations. This means that we cannot generalise the findings to real life.

One limitation of Miller's research (1956)

One limitation is that they may have overestimated the STM capacity. Cowan (2001) reviewed other pieces of research and concluded that the STM capacity may only be about 4 plus or minus 1 chunks. This suggests that the lower end of Miller’s estimate is more appropriate than 7 items.

What is the capacity of LTM?

It has an unlimited capacity.