126 • CHAPTER 5 Short-Term and Working Memory
According to measurements of digit span, the average
capacity of STM is about 5 to 9 items—about the length of
a phone number. This idea that the limit of STM is some-
where between 5 and 9 was suggested by George Miller
(1956) in a famous paper titled “The Magical Number
Seven, Plus or Minus Two.” In that paper, Miller summa-
rized evidence suggesting that STM can hold 5 to 9 items.
More recent measures of STM capacity have set the capac-
ity at about 4 items (Cowan, 2001). This conclusion is based
on the results of experiments like the one by Steven Luck and
Edward Vogel (1997), which measured the capacity of STM
by fl ashing two arrays of colored squares separated by a brief
delay (● Figure 5.8a). The participants’ task was to indicate
whether the second array was the same as or different from the
fi rst array. On trials in which the second array was different,
the color of one square was changed, as shown in Figure 5.8a.
The result of this experiment, shown in Figure 5.8b,
shows that performance was almost perfect when there
were 1 to 3 squares in the arrays, but that performance
began decreasing when there were 4 or more squares. Luck
and Vogel concluded from this result that participants were
able to retain about 4 items in their short-term memory.
Other experiments, using verbal materials, have come to
the same conclusion (Cowan, 2001).
These estimates of either 4 items or 5 to 9 items set
rather low limits on the capacity of STM. If our ability to
hold items in memory is so limited, how is it possible to
hold many more items in memory in some situations, as
when words are arranged in a sentence? The answer to this
question was proposed by Miller, who introduced the idea
of chunking in his 7-plus-or-minus-2 paper.Chunking Miller (1956) introduced the concept of
chunking to describe the fact that small units (like words)
can be combined into larger meaningful units, like phrases, or even larger units, like
sentences, paragraphs, or stories. Consider, for example, trying to remember the fol-
lowing words: monkey, child, wildly, zoo, jumped, city, ringtail, young. How many
units are there in this list? There are 8 words, but if we group them differently, they
can form the following 4 pairs: ringtail monkey, jumped wildly, young child, city zoo.
We can take this one step further by arranging these groups of words into one sen-
tence: The ringtail monkey jumped wildly for the young child at the city zoo.
A chunk has been defi ned as a collection of elements that are strongly associated with
one another but are weakly associated with elements in other chunks (Cowan, 2001;
Gobet et al., 2001). In our example, the word ringtail is strongly associated with the word
monkey but is not as strongly associated with the other words, such as child or city.
Thus, chunking in terms of meaning increases our ability to hold information in
STM. We can recall a sequence of 5 to 8 unrelated words, but arranging the words to
form a meaningful sentence so that the words become more strongly associated with
one another increases the memory span to 20 words or more (Butterworth et al., 1990).
Chunking of a series of letters is illustrated by the following demonstration.DEMONSTRATION Remembering Letters
Read the string of letters below at a rate of about 1 letter every 2 seconds, and then cover the
letters and write down as many as you can, in the correct order:
B C I F N C C A S I C B● FIGURE 5.8 (a) Stimuli used by Luck and Vogel (1997). The
participant sees the fi rst display and then indicates whether the
second display is the same or diff erent. In this example, the color
of one square is changed in the second display. (b) Result of the
experiment, showing that performance began to decrease once
there were 4 squares in the display. (Source: Adapted from E. K. Vogel,
A. W. McCollough, & M. G. Machizawa, “Neural Measures Reveal Individual Differences in
Controlling Access to Working Memory,” Nature 438, 500–503, 2005.)0 4 8 125075100Number of squares100 ms
(a)(b)900 ms delay 2,000 ms
Same or different?Percent correctCopyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.