Appraisal does not work in a vacuum. Not only does it embody certain political assumptions; it is itself, part of the political process. But this does not define the way in which cost-benefit or any other formal appraisal technique will actually be applied. Appraisal is part of a larger planning or decision process. Most public sector organisations are big, or they operate through systems which include a large number of interacting individuals and institutions, operating under strict rules of procedure. The assets with which they are concerned, particularly in transport, are large, expensive and long lived. All these factors go to determine the way in which cost-benefit analysis, or any other method of appraisal, can be used; they determine its precise role and function in practice. Cost-benefit analysis can be used more or less intensively, more or less expensively, more or less precisely and in more or less detail. How it should be used depends very much on the particular purpose of appraisal which in turn depends on context. Without a definition of context, the validity or true function of a given approach cannot be properly assessed.