Later research on short-term memory and working memory revealed that memory span is not a constant even when measured in a number of chunks. The "magical number 7" and working memory capacity The number seven constitutes a useful heuristic, reminding us that lists that are much longer than that become significantly harder to remember and process simultaneously. Nevertheless, the idea of a "magical number 7" inspired much theorizing, rigorous and less rigorous, about the capacity limits of human cognition. Therefore, there is nothing "magical" about the number seven, and Miller used the expression only rhetorically. Miller recognized that the correspondence between the limits of one-dimensional absolute judgment and of short-term memory span was only a coincidence, because only the first limit, not the second, can be characterized in information-theoretic terms (i.e., as a roughly constant number of bits). For instance, a word is a single chunk for a speaker of the language but is many chunks for someone who is totally unfamiliar with the language and sees the word as a collection of phonetic segments. A chunk is the largest meaningful unit in the presented material that the person recognizes-thus, what counts as a chunk depends on the knowledge of the person being tested. Miller concluded that memory span is not limited in terms of bits but rather in terms of chunks.
He noticed that memory span is approximately the same for stimuli with vastly different amounts of information-for instance, binary digits have 1 bit each decimal digits have 3.32 bits each words have about 10 bits each. Miller observed that the memory span of young adults is approximately seven items. Memory span refers to the longest list of items (e.g., digits, letters, words) that a person can repeat back in the correct order on 50% of trials immediately after the presentation. The second cognitive limitation Miller discusses is memory span. Therefore, people's maximum performance on a one-dimensional absolute judgment can be characterized as an information channel capacity with approximately 2 to 3 bits of information, which corresponds to the ability to distinguish between four and eight alternatives. The information contained in the input can be determined by the number of binary decisions that need to be made to arrive at the selected stimulus, and the same holds for the response. The task can be described as one of information transmission: The input consists of one out of n possible stimuli, and the output consists of one out of n responses. Performance is nearly perfect up to five or six different stimuli but declines as the number of different stimuli increases. In a one-dimensional absolute-judgment task, a person is presented with a number of stimuli that vary on one dimension (e.g., 10 different tones varying only in pitch) and responds to each stimulus with a corresponding response (learned before).
In his article, Miller discussed a coincidence between the limits of one-dimensional absolute judgment and the limits of short-term memory. 2 The "magical number 7" and working memory capacity.
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