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Donald A. Norman was previously head of the Apple Research Laboratories, and Professor of Cognitive Science at UCSD.

He, (with Jakob Nielson and Tog) is one of the principals of the Nielson Norman Group - (usability gurus).

Norman can be described as an intellectually turbocharged Grandpa Smurf.

Norman was a pioneer of user centred design, a philosophy which places the responsibility for usability clearly with the designer of products, whether they be web sites or aircraft controls. Products, says Norman, must be designed to allow for the humanity of their expected users. Designers must be aware of the limits imposed by our sensory systems (which vary as we age), our physical abilities (which vary for all sorts of reasons), and, most importantly, the way human beings think.

In his book "The Design of Everyday Things", first published in 1988 and a classic text, Norman examines these areas in detail, and covers many examples of poor and good design; everything from doors to the control panels of nuclear power stations. One of his most powerful concepts is the delineation of the three aspects of mental models relevant to the design and use of any product. I'll do my best to explain:

The Design Model
First, the designer's mental model is the designer's understanding of the nature and operational structure of the problem. It is the designer's mental image of the "problem space".

The System Model or Image
In an attempt to solve this problem, the designer creates a product which encapsulates an analogue of the design image, and allows the user to operate on the product to bring about the desired set of outcomes. The mapping of the design model into the product gives the system image.

The User's Model
When a user tries to make use of the product, she brings with her a set of expectations based on previous experience. When a user and a product meet, any number of user models may be born, depending on the psychology and background of the user, and the visibility (obviousness) and appropriateness of the system image. The user experience will be frustrating if the product produces unexpected results, or the expected result in unexpected ways, or demands inputs which require taxing the user's physical or mental resources unnecessarily (and the list continues). If the user's mental model and the system image coincide, the user will use the product "correctly", and the experience will be as good as possible, given the limits of the designer's skill and the mundanity of the task.

An Example
I can't think of a better example than the one Norman uses: the thermostat for a central heating system.

Design Image: There is i) a controllable (on/off) heat source, ii) a defined volume A which needs to be temperature controlled, iii) B, a volume outside of this (the rest of the Universe, in fact, but don't worry about this.), iv) a device allowing a user to specify the desired temperature of volume A, and v) a user.

Assuming we are dealing only with heating, not cooling, the design model is simple - allow the user to specify a desired temperature T. If the actual temperature is below T, then turn on the heat source, and keep checking the temperature, and if or when T is reached, turn off the heat source. If the actual temperature is above T, then keep the heat source inactive.

System Image: Present the user with a device with a scale of some kind, allowing her to select the desired temperature. Some thermostats also show if the heat source is on or off. The device contains a switch, turning the heat source on and off depending on the desired and actual temperatures, as described above.

User Model: This may be the same as the system image, i.e. a temperature based on/off switch, but for many people the user model is analogous to a valve - the higher the temperature on the thermostat is set, the faster heat is generated, and so the faster the temperature will rise. Other than marginal cases, where feedback errors produce slight complications, this model is completely incorrect. It will lead to people setting the thermostat inappropriately, and may result in over or under heating, discomfort, and energy wastage. It is also the most common user model of thermostats. Norman calls this the "folk theory" of thermostats.

The point is not that the user has an incorrect model, but that users will always construct a mental model, and it is up to the designer to ensure that this coincides with the system image. This may be through the physical shape or other attributes of the product, or through marks on it, e.g. graphics or text.

This example is perhaps trivial (though environmentalists would disagree), but the same principals apply to all controls, including aircraft, weapons, automobiles and power stations. Mistakes made controlling these have all resulted in deaths. Add to this the daily irritation we all feel when thwarted or obstructed by the devices (and web sites) we use every day, and you can see the importance of Norman's ideas, of which this node is only a tiny and selective glimpse.

Information Appliances
Norman has also written about the need that typical people have for information appliances. These are devices designed to meet specific and limited information delivery requirements, as opposed to trying to get one device to do everything, as in the PC. The market for information appliances is currently much greater than that for PCs, and is growing. Currently popular information appliances include PDAs, mobile phones, MP3 players, games consoles and video recorders.

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