capacity

• Digit span was recognised by Joseph Jacobs in 1887. It requires calling out 4 digits and seeing if the participants can recall them in the same number and if they can they add another digit each time until they cannot recall them correctly. The average capacity is 7+/-2.
• George Miller 1956 realised that things normally come in sevens. He also recognised that people normally chunk things together as it is easier to remember. 
• One strength of Jacobs' study is that it had been replicated. The study is very old and with the earliest studies in psychology they often lack adequate controls. As some of the participants' digit span may have been underestimated because they may have been distracted during testing which is a confounding variable. Despite this it has been proved by other psychologists in more controlled conditions. This suggests that it is a valid test for the STM digit span. 
• One limitation in Miller’s research is that he overestimated the STM capacity. Ads Nelson Cowan in 2001 reviewed the research and concluded the capacity is around 4+/-1.This suggests that Miller's research would have been more appropriate with 5 items and not seven.