The Top-Down Approach to Functional Analysis

The first central issue in functional analysis is then Starting out with a complex requirements definition document (RDD) (i.e. with numerous and interacting requirements on the functionality), how do we select a set of functional elements and their interactions such that the resulting functional system satisfies the RDD We could try to pick a subset from a set of previously used elements and let them interact in a particular fashion. But we are faced with exactly the same problem as we encountered with the bottom-up approach to design in the physical domain, as discussed in Sec. B5.2. It is unlikely that the emergent behaviour of the functional system will meet all the requirements in the RDD. It will require a number of iterations to achieve that, and even then we have no way of knowing and demonstrating that we have found the best solution (the second central issue in functional analysis, to be discussed in the next section). 

The answer to the question is, in my opinion, yes. In Sec. B2.4 we put forward the following argument Every engineering project involves the expenditure of resources in some form as labour, as materials, as natural resources, and so on. As a result, there is a cost associated with every project. It may not always be explicitly accounted for, e.g. if it is in the form of voluntary labour or a gift, but it is a cost attributable to the project all the same, in the sense of opportunity cost. But nobody would expend resources without the expectation of some form of revenue. Again, this may not always be accounted for directly in monetary terms  

The cost precedes the revenue, at least to some extent, and so it is in the nature of an investment, and the quantity of interest is the revenue relative to the investment, or the Return on Investment (ROI). The exact definition of this relationship in accounting terms may vary somewhat between various application domains and, in particular, due to the different compositions of the stakeholder groups. This will be discussed in Sec. C4.5, but for the time being the simplest relationship, such as revenue minus cost, divided by cost, is an adequate definition of ROI. The function of generating a return on investment then becomes the universal top element in developing a functional system the starting point of design in the functional domain, and what we shall call the irreducible element. 

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