Three Elements of Good Engineering Design

Practically, all engineering design problems require three (3) elements to come up with a complete solution.

1. Measurable Elements.
These elements can be determined with exactness. Example - material costs, strength of parts of construction, load tests, etc.
2. Assumed Elements.
These elements can not be determined with exact measurement or computation. These are usually a development from practical experience over a number of years and are often matters of accepted standard practice. Example - probable increase in load, allowable voltage regulation, safety factors, standards of construction, etc. To determine these elements, empirical methods must here be employed.
3. Good Judgment.
This is the most intangible among all elements. The good judgment of the engineer based on his knowledge and experience and applied to the particular problem, to be able to utilize the first two elements in order to create an efficient and economical design best adapted to the situation.

All of these elements in their proper proportions are equally important in attaining the most satisfactory solution for any problem. Empirical methods if applied blindly may divert from the purpose for which they were originally intended.

Judgment based on experience alone is liable to become a mere guess unless supported by exact knowledge and good practice. It would appear that the further the exact data can be carried in a problem, the less dependence need be placed on the more intangible elements, hence the greater certainty of the best solution.

This exact knowledge is the main purpose of the study of the economic features of a problem. In many cases economy may be made the deciding factor between two or more designs apparently equally good from other points of view

Reference: Economics of Electrical Distribution - P. O. Reyneau