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mappings reduce the information load on the user. The standard measure of information is the “bit,” the amount of information required to distinguish between two items. With the completely arbitrary mapping of figure 3.3, each control could work any of four burners, so it takes 2 bits of information to specify which burner each control operates. If you wish to be able to look at any of the four controls and know immediately which burner it operates, 8 bits must be learned. Eight bits is a lot. (Technically, all four controls can be specified with a total of only 4.6 bits, but this makes use of the fact that once the first control is known [2 bits], the second one has to be selected from only three possibilities [1.5 bits], the third from the two remaining possibilities [1 bit], and then the last control becomes fully determined [o bits]. This strategy requires less information to specify all four controls, but at the cost of more computation: you can’t look directly at a control and know which burner it works—you have to figure it out.)

The partial mapping of arrangement figure 3.4 reduces the information load. Now, selection of each proper control is selection from two alternatives, or 1 bit. So a total of only 4 bits are required to let the person go to each control and know immediately which burner it operates. The full natural mappings of figure 3.5 have only one interpretation, so nothing has to be learned: o bits.

The change from an arbitrary mapping to a partial mapping to a full natural mapping reduces the number of alternatives from 24 to 4 to 1 and reduces the information theory content from 8 to 4 to o bits, respectively.

17 Despite the importance of reminding from both the practical and the theoretical point of view, little is known about it. Reminds, of course, occur in a number of different ways. One form of reminding occurs entirely internally, as when a thought or an experience “reminds” one of another thought or experience. As far as I know, only Roger Schank has written about it (in his book Dynamic memory, 1982). Another form of reminding comes from external cues: for example, as when seeing a clock reminds one of the time and of a task that needs to be done (or worse, that can no longer be done). Another form of reminding—the type I have been discussing—is deliberately invoked or set up, as when one tries to set up physical cues on one day of tasks that are to be done during another. We treat some of these issues in the chapters by Cypher and by Miyata and Norman in Norman & Draper (1986), User centered system design.

CHAPTER FOUR: Knowing What to Do

1 Letter to newspaper advice columnist Ellie Rucker, Austin [Texas] American-Statesman, Aug. 31, 1986. Reprinted by permission.

2 The results of my experiments are reminiscent of studies of chess masters who were allowed only ten seconds to examine a chess board with a configuration from the middle of a real game before being asked to reconstruct the board from memory. They did so very accurately. Novices reconstruct the board poorly. But show an illegal (or illogical) combination of those very same chess pieces to a master and a novice, and they perform about equally poorly. The expert has learned so much of the structure of the game that numerous natural and artificial constraints come into play, automatically ruling out all sorts of configurations and reducing what has to be remembered to a manageable amount. The novice does not have sufficient internal knowledge to make use of these constraints. Similarly, when faced with the illegal or illogical configuration, the expert’s constraints and prior knowledge are no longer useful (see Chase & Simon, 1973).

3 See Schank & Abelson’s (1977) Scripts, plans, goals, and understanding or Goffman’s (1974) book Frame analysis, on social structures and conventions.

4 We had to overcome a number of technical problems to improve the mapping. The lights were already installed, and it was not possible to redo the wiring. We modified some light dimmers so that they could be used as controls for remotely