Chunks and Chunking

How much information does short-term memory contain at any one time? You feel subjectively that you are consciously aware of a lot of things at the same time all the different parts of your visual field (what you see before you), for example. But most of this feeling reflects filling in rather than direct experience. Measurement of your memory span (also called span of attention or span of apprehension) shows that you can keep only about seven things at a time in your conscious short term memory.

Memory span is defined as the number of separate items (letters, digits, words, etc.) that you can repeat back immediately without error. Most people can repeat about seven digits or letters perfectly almost every time. For example, as the Figure illustrates, most people easily recall all six letters of a 6- letter display they see for about a second. Many people can recall all seven letters of a 7-letter display, but only a few can consistently report all eight letters of a 8-letter display.

Figure 1. Measurement of memory span. The approximate % of people who recall all letters in increasingly longer strings of letters shown for about a second.

But why can you read "ice hockey," if short term memory holds only seven letters? Because you don't treat "ice hockey" as nine letters and a space; you treat it as two words, or, more likely, a single idea. You recode the letters into words and the ideas behind the words. This recoding process is often called chunking, and the idea you recode it as is called a chunk (G. Miller, 1956). It is the chunk that forms a single item in short term memory.

For example, the string of digits 0 1 1 0 0 1 0 1 1 1 0 0 1 1 1 far exceeds memory span, but you can recode into 3 1 3 4 7 using the binary to decimal conversion:

    0 0 0 = 0;    0 0 1 = 1;    0 1 0 = 2;    0 1 1 = 3;    1 0 0 = 4;    1 0 1 = 5;    1 1 0 = 6;    1 1 1 = 7

Another example: 1 4 9 2 1 7 7 6 1 8 6 0 1 9 1 4 1 9 4 1 is far beyond memory span, but you can easily recode this string of digits as important dates in U. S. history.

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