The Magical Number 7--or is it 4?

[Laurence Horn]

Relating to our earlier thread on numbers and memory...

>        Below is the abstract of a forthcoming BBS target article
>
>This article has been accepted for publication in Behavioral and Brain
>Sciences (BBS), an international, interdisciplinary journal providing
>Open Peer Commentary on important and controversial current research in
>the biobehavioral and cognitive sciences.
>
>_____________________________________________________________
>
>    THE MAGICAL NUMBER 4 IN SHORT-TERM MEMORY:
>    A RECONSIDERATION OF MENTAL STORAGE CAPACITY
>
>       Nelson Cowan
>       Department of Psychology
>       University of Missouri
>       210 McAlester Hall
>       Columbia, MO 65211, USA
>       CowanN at missouri.edu
>       http://web.missouri.edu/~psycowan
>
>    ABSTRACT: Miller (1956) summarized evidence that people can
>    remember about 7 chunks 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. Others have since suggested
>    that there is a more precise capacity limit, but that it is only 3
>    to 5 chunks. The present target article brings together a wide
>    variety of data on capacity limits suggesting that the smaller
>    capacity limit is real. Capacity limits will be useful in analyses
>    of information processing only if the boundary conditions for
>    observing them can be carefully described. Four basic conditions in
>    which chunks can be identified and capacity limits can accordingly
>    be observed are: (1) when information overload limits chunks to
>    individual stimulus items, (2) when other steps are taken
>    specifically to block the recoding of stimulus items into larger
>    chunks, (3) in performance discontinuities caused by the capacity
>    limit, and (4) in various indirect effects of the capacity limit.
>    Under these conditions, rehearsal and long-term memory cannot be
>    used to combine stimulus items into chunks of an unknown size; nor
>    can storage mechanisms that are not capacity-limited, such as
>    sensory memory, allow the capacity-limited storage mechanism to be
>    refilled during recall. A single, central capacity limit averaging
>    about 4 chunks is implicated along with other, non-capacity-limited
>    sources. The pure STM capacity limit expressed in chunks is
>    distinguished 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 proposed.