p.xi This book is about systems. It focuses on the engineering
of systems and on systems analysis.
p.3 The Elements of a System [section title] Systems are composed of components, attributes, and relationships.
These are described as follows: 1. Components are the operating parts of a system consisting of
input, process, and output. Each system component may assume a variety of values to describe a system state as set by some
control action and one or more restrictions. 2. Attributes are the properties of discernible manifestations
of the components of a system. These attributes characterize the system. 3. Relationships are the
links between components and attributes.
A system is a set of interrelated components working together toward some common objective or purpose.
p.5 Constraints placed on the system limit its operation and define the boundary within which it is
intended to operate.
p.7 Physical systems are those that manifest themselves in physical form. They are composed of
real components and may be contrasted with conceptual systems, where symbols represent the attributes of components.
p.16 Engineering activities of analysis and design for human-made
or technical systems are not an end in themselves but are a means for satisfying human wants. Thus, modern engineering
has two aspects. One aspect concerns itself with the materials and forces of nature; the other is concerned with the needs
of people.
p.18 [Systems Engineering is] An approach to translate operational needs and requirements into operationally
suitable blocks of systems. The approach shall consist of a top-down, iterative process of requirements analysis,
functional analysis and allocation, design synthesis and verification, and system analysis and control. Systems engineering
shall permeate design, manufacturing, test and evaluation, and support of the product. Systems engineering principles shall
influence the balance between performance, risk, cost, and schedule.
p.55-58 PROBLEM DEFINITION AND NEED IDENTIFICATION [section title] The systems engineering
process generally commences with the identification of a "want" or "desire" for something and is based on a
real (or perceived) deficiency. For instance, suppose that a current system capability is not adequate in terms
of meeting certain required performance goals, is not available when needed, cannot be properly supported, or is too costly
to operate... Defining the problem is sometimes the most difficult part of the process... Having justified
the need for a new system, it is then necessary to (1) identify various possible system-level design approaches that could
be pursued in response to the need; (2) evaluate the most likely approaches in terms of performance, effectiveness, maintenance
and sustaining support, and life-cycle economic criteria; and (3) recommend a preferred course of action. There may be
many possible alternatives; however, the number must be narrowed down to a few feasible ones, consistent with the schedule
requirements and the availability of resources
p.109 A model, in this context, is a simplified representation of the real world which abstracts
features of the situation relative to the problem being analyzed... The model used should incorporate the following
features.
1. The model should represent the dynamics of the system configuration being evaluated
in a way that is simple enough to understand and manipulate, and yet close enough to the operating reality to yield successful
results.
2. The model should highlight those factors that are most relevant to the problem at hand,
and suppress (with discretion) those that are not as important.
3.The model should be comprehensive by including all relevant factors and be reliable
in terms of repeatability of results.
4.Model design should be simple enough to allow for timely implementation in problem solving...
5.Model design should incorporate provisions for ease of modification or expansion to permit
the evaluation of additional factors as required.
p.162-163 An understanding of the decision-making process usually requires
simplification of the complexity facing the decision maker. Conceptual simplifications of reality, or models, are a means
to this end... Those factors that stand in the way of attaining objectives are known as limiting factors.
An important element of the systems engineering process is the identification of the limiting factors restricting accomplishment
of a desired objective. Once the limiting factors have been identified, they are examined to locate strategic
factors, those factors that can be altered to make progress possible.
The identification of strategic factors is important, for it allows the
decision maker to concentrate effort on those areas in which success is obtainable. This may require inventive ability, or
the ability to put known things together in new combinations, and is distinctly creative in character. The means that will
achieve the desired objective may consist of a procedure, a technical process, or a mechanical, organizational, or managerial
change. Strategic factors limiting success may be circumvented by operating on engineering, human, and economic factors individually
and jointly.
p.164 To compare alternatives equivalently, it is important that they be converted to a common measure.
This conversion to a common measure permits comparison on the basis of equivalence.... decisions between alternatives should
be made on the basis of their differences. Thus, all identical factors can be canceled out for the comparison of any two or
more alternatives at any step of a decision-making process.
p.164-165 Models and their manipulation (the process of simulation) are useful tools in systems analysis...
Models are designed to represent a system under study, by an idealized example of reality, to explain the essential
relationships involved.
p.168 Because of the impossibility of including all factors in constructing the evaluation function [Blanchard
and Fabrycky are speaking of a general mathematical decision model], it is common practice to consider only those on which
the outcome is believed to depend significantly... Although it may be feasible to consider only those relationships
that are significantly pertinent, one should remember that all system elements are interdependent.
p.169 Decision evaluation is an important part of systems engineering and analysis. Evaluation is needed
as a basis for choice among alternatives that arise from design activities, as well as for optimizing systems already in operation.
In either case, equivalence provides the common evaluation measure on which choice can be based.
p.187 A basic human tendency is to focus on the most probable outcome from among several that could occur.
This approach to decision making suggests that all except the most probable future be discarded... this criterion works well
when the most probable future has a significantly high probability so as to partially dominate.
