Capacity of Short-term Memory
Whatever the cause or causes of forgetting over the short-term may be, there is consensus that it severely limits the amount of new information that we can retain over brief periods of time. This limit is referred to as the finite capacity of short-term memory. The capacity of short-term memory is often called memory span, in reference to a common procedure of measuring it. In a memory span test, the experimenter presents lists of items (e.g. digits or words) of increasing length. An individual's span is determined as the longest list length that he or she can recall correctly in the given order on at least half of all trials.
In an early and highly influential article, The Magical Number Seven, Plus or Minus Two, the psychologist George Miller suggested that human short-term memory has a forward memory span of approximately seven items plus or minus two and that that was well known at the time (it seems to go back to the 19th-century researcher Wundt). More recent research has shown that this "magical number seven" is roughly accurate for college students recalling lists of digits, but memory span varies widely with populations tested and with material used. For example, the ability to recall words in order depends on a number of characteristics of these words: fewer words can be recalled when the words have longer spoken duration; this is known as the word-length effect, or when their speech sounds are similar to each other; this is called the phonological similarity effect. More words can be recalled when the words are highly familiar or occur frequently in the language. Recall performance is also better when all of the words in a list are taken from a single semantic category (such as games) than when the words are taken from same categories. According to the available evidence, the best overall estimate of short-term memory is about four pieces or "chunks" of information. In free recall it has been shown, to the contrary, that there is no such "quantized" limit, rather it is a function of memory decaying with time.
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