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.
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
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"
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
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
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.