The magical number 4 in short-term memory: A reconsideration of mental storage capacity

Miller (1956) summarized evidence that people can remember about seven chun ks in short-term memory (STM) tasks. However, that number was meant more as a rough estimate and a rhetorical device than as a real capacity limit. Ot hers have since suggested that there is a more precise capacity limit, but that it is only three to five chunks. The present target article brings tog ether a wide variety of data on capacity limits suggesting that the smaller capacity limit is real. Capacity limits will be useful in analyses of info rmation processing only if the boundary conditions for observing them can b e carefully described. Four basic conditions in which chunks can be identif ied and capacity limits can accordingly be observed are: (1) when informati on overload limits chunks to individual stimulus items, (2) when other step s are taken specifically to block the recoding of stimulus items into large r chunks, (3) in performance discontinuities caused by the capacity limit, and (4) in various indirect effects of the capacity limit. Under these cond itions, rehearsal and long-term memory cannot be used to combine stimulus i tems into chunks of an unknown size; nor can storage mechanisms that are no t capacity-limited, such as sensory memory, allow the capacity-limited stor age mechanism to be refilled during recall. A single, central capacity limi t averaging about four chunks is implicated along with other, noncapacity-l imited sources. The pure STM capacity limit expressed in chunks is distingu ished from compound STM limits obtained when the number of separately held chunks is unclear. Reasons why pure capacity estimates fall within a narrow range are discussed and a capacity limit for the focus of attention is pro posed.