Spoken with almost-audible quotation marks such a statement may take on the authority of holy writ, and therein lies a danger. The danger is that a particular heuristic may have been regarded so favorably by practitioners that it is taken to be a fundamental truth. The trouble is that times change and, particularly in computing, so does technology. So that a design heuristic that has been used successfully for some time may rapidly become obsolete and untrustworthy.
Despite these difficulties, design could not take place at all without heuristics. You will find soon that design strategy usually makes use of them, and they also play an important part in ‘polishing’ or refining a first design acquired by other techniques. Thus part of a designer’s skill lies in his or her ability, not only to apply heuristics, but also to choose them with care.
1.3 DESIGN CRITERIA
We commenced sec. 1.2 with a simple problem, designing the transportation arrangements for a family day-out to the seaside. We noted that with three alternatives and a single criterion, direct costs or journey time, the problem is trivial. But we also stated that if both criteria are important then the design problem becomes rather more difficult. To take this matter a little further, let us suppose that the parent decision-maker, from the information available about fare structures, timetables and so on, has assembled the table shown as Fig.1.3.
The three modes of transport are called P, Q and R to disguise our own prejudices. For each mode, estimates of direct costs and journey time are entered. It is clear that if direct cost is the sole criterion then R is favorite; if journey time is of lone importance then P wins, hands down. Try and imagine yourself in the position of the parent decision-maker. On the basis of the information in Fig.1.3, what would your choice be? (Just for once we are not concerned with your actual answer but only with how you arrived at it.)
If you arrived at a decision, and whether your answer was P, Q or R, you almost certainly got there by using the process of trading off, although you may not have been aware that you were so doing.
You probably posed yourself questions such as: ‘is it worth incurring extra costs of £2.35 in order to save 25 minutes on the journey?’ and from the answers that you gave yourself, you arrived at your conclusion.
You may also have thought that it was a daft question for, in practice, you would need to take other properties or attributes into account, e.g., comfort, convenience and if Aunt Gloria insists on coming (as she usually does), the scenic beauty of the route, perhaps. You may have noted at the same time that these three attributes of the transport mode are intangible, so that trading them off against direct costs is a non-starter anyway.
You may also have toyed with ideas of uncertainty, bearing in mind that engineering works, traffic jams, staff shortages and breakdowns could completely invalidate the stated journey times. These complaints are quite reasonable but, nevertheless, people do take decisions like the one we have been discussing here, all the time. Largely, they will use a combination of trading off and
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