Page 294 - Mechanical Engineers' Handbook (Volume 2)

5 System Design 285

scale issues is very small compared with the size and scope of the issues whose resolution
is required for objective, substantive, and procedurally rational behavior. Among the limits
to rationality are the fact that we can formulate, analyze, and interpret only a restricted
amount of information; can devote only a limited amount of time to decision making; and
can become involved in many more activities than we can effectively consider and cope with
simultaneously. We must therefore necessarily focus attention only on a portion of the major
competing concerns. The direct effect of these is the presence of cognitive bias in information
acquisition and processing and the use of cognitive heuristics for evaluation of alternatives.
Although in many cases these cognitive heuristics will be flawed, this is not necessarily
so. One of the hoped-for results of the use of systems engineering approaches is the devel-
opment of effective and efficient heuristics for enhanced judgment and choice through ef-
fective decision support systems. 30
There are many cognitive biases prevalent in most information acquisition activities.
The use of cognitive heuristics and decision rules is also prevalent and necessary to enable
us to cope with the many demands on our time. One such heuristic is satisfying or searching
for a solution that is ‘‘good enough.’’ This may be quite appropriate if the stakes are small.
In general, the quality of cognitive heuristics will be task dependent, and often the use of
heuristics for evaluation will be both reasonable and appropriate. Rational decision making
requires time, skill, wisdom, and other resources. It must, therefore, be reserved for the more
important decisions. A goal of systems engineering is to enhance information acquisition,
processing, and evaluation so that efficient and effective use of information is made in a
process that is appropriate to the cognitive styles and time constraints of management.

5 SYSTEM DESIGN
This section discusses several topics relevant to the design and evaluation of systems. In
order to develop our design methodology, we first discuss the purpose and objectives of
systems engineering and systems design. Development of performance objectives for quality
systems is important, since evaluation of the logical soundness and performance of a system
can be determined by measuring achievement of these objectives with and without the sys-
tem. A discussion of general objectives for quality system design is followed by a presen-
tation of a five-phase design methodology for system design. The section continues with
leadership and training requirements for use of the resulting system and the impact of these
requirements upon design considerations. While it is doubtless true that not every design
process should, could, or would precisely follow each component in the detailed phases
outlined here, we feel that this approach to systems design is sufficiently robust and generic
that it can be used as a normative model of the design process and as a guide to the
structuring and implementation of appropriate systems evaluation practices.

5.1 The Purposes of Systems Design
Contemporary issues that may result in the need for systems design are invariably complex.
They typically involve a number of competing concerns, contain much uncertainty, and
require expertise from a number of disparate disciplines for resolution. Thus, it is not sur-
prising that intuitive and affective judgments, often based on incomplete data, form the usual
basis used for contemporary design and associated choice making. At the other extreme of
the cognitive inquiry scale are the highly analytical, theoretical, and experimental approaches
of the mathematical, physical, and engineering sciences. When intuitive judgment is appro-
priately skill based, it is generally effective and appropriate. One of the major challenges in