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6.3 Some practical issues 179

6.3.4 How do you choose among alternative designs?
Choosing among alternatives is about making design decisions: Will the device use
keyboard entry or a touch screen? Will the device provide an automatic memory
function or not? These decisions will be informed by the information gathered
about users and their tasks, and by the technical feasibility of an idea. Broadly
speaking, though, the decisions fall into two categories: those that are about exter-
nally visible and measurable features, and those that are about characteristics in-
ternal to the system that cannot be observed or measured without dissecting it.
For example, externally visible and measurable factors for a building design in-
clude the ease of access to the building, the amount of natural light in rooms, the
width of corridors, and the number of power outlets. In a photocopier, externally
visible and measurable factors include the physical size of the machine, the speed
and quality of copying, the different sizes of paper it can use, and so on. Underly-
ing each of these factors are other considerations that cannot be observed or stud-
ied without dissecting the building or the machine. For example, the number of