In this study, projects are not regarded as isolated economic units; they are considered within the context of the entire economic system of which they will form a part. Thus the Manual presents both macroeconomic and microeconomic concepts. From this it must not be inferred that the Manual attempts to offer a combined microeconomic and macroeconomic theory. It seeks to contribute more to an appreciation of the problem than to its solution, thus widening the outlook of those who prepare projects so that they may make the greatest possible compilation of useful data for their economic appraisal.