Capacity of the STM
Joseph Jacobs used the digit span technique to measure the capacity of the short term memory. He found the average span for digits was 9.3 items and 7.3 for letters- he suggested this is because there are more letters (26) than numbers (9).
Miller reviewed Jacobs' research and cocnluded that the immediate memory span is 7+/-2 items. Miller also recognised that people can remember 5 letters as well as they can 5 words- suggesting that chunking information makes it easier to remeber.