The challenge has come from results such as those from Chen and Cowan, in which the predicted results from an experiment were that immediate serial recall of absolute numbers of singleton words would be the same as the number of chunks of learned pair words.
However in fact it was found that the same number of pre-exposed singleton words was recalled as the number of words within learned pairs - eg 8 words (presented as 8 singletons or 4 learned pairs).
The propensity to use rehearsal and memory aids is a serious complication in accurately measuring the capacity of short-term memory.
Indeed many of the studies ostentatiously measuring short-term memory capacity have been argued to be actually measuring the ability to rehearse and access long-term memory stores (Cowan, 2001).
Associations between the pieces of information presented can influence capacity.
Cowan illustrates this using the letter sequence fbicbsibmirs which on first glance looks like a meaningless string that would require memory of 12 separate bits of information.In this participants had to recall information read aloud to them and results clearly showed that there was a normal upper limit of 9 when binary items were used.This was despite the constant information hypothesis, which has suggested that the span should be long if each presented item contained little information (Miller, 1956).The historical view of short-term memory capacity The span of absolute judgement is defined as the limit to the accuracy with which one can identify the magnitude of a unidimensional stimulus variable (Miller, 1956), with this limit or span traditionally being around 7 2.Miller cites Hayes memory span experiment as evidence for his limiting span.However it is not a simple case of being able to remember large chunks immediately, rather that as each bit becomes more familiar, it can be assimilated into a chunk, which is then remembered itself.Recoding is the process by which individual bits are ‘recoded’ and assigned to chunks.However 6 learned pairs could be recalled as successfully as 6 pre-exposed singleton words (Chen & Cowan, 2005).This suggested a different mechanism for recall depending on the circumstances.Short-term memory is the memory for a stimulus that lasts for a short while (Carlson, 2001).In practical terms visual short-term memory is often used for a comparative purpose when one cannot look in two places at once but wish to compare two or more possibilities.