Source: NIH website, Feb 2010
Working memory storage capacity is important because cognitive tasks can be completed only with sufficient ability to hold information as it is processed. The ability to repeat information depends on task demands but can be distinguished from a more constant, underlying mechanism: a central memory store limited to 3 to 5 meaningful items in young adults.
Many studies indicate that working memory capacity varies among people, predicts individual differences in intellectual ability, and changes across the life span (Cowan, 2005).
As Cowan (2001) noted, many theorists with mathematical models of particular aspects of problem-solving and thought have allowed the number of items in working memory to vary as a free parameter, and the models seem to settle on a value of about 4, where the best fit is typically achieved.
The capacity-limit-as-strength camp includes diverse hypotheses. Mathematical simulations suggest that, under certain simple assumptions, searches through information are most efficient when the groups to be searched include about 3.5 items on average. A list of three items is well-structured with a beginning, middle, and end serving as distinct item-marking characteristics; a list of five items is not far worse, with two added in-between positions. More items than that might lose distinctiveness within the list. A relatively small central working memory may allow all concurrently-active concepts to become associated with one another (chunked) without causing confusion or distraction.