Insufficiency of the data necessary for development planning is one of the characteristics of economic underdevelopment. Even a minimal coordination of efforts is impossible without a minimum of information on the technical and economic environment, costs and possible benefits of alternative courses of action, and the specific investment opportunities. Put simply: it is not possible to decide what is yet to be done if one does not know what has already been done.
Adequate programming can take place without a completely detailed picture and, even in the least developed of countries, some data is always available. In a development project, it is necessary to determine the extent of the gap between the set of data required and the set of data actually available in the economy. The practical definition of the 'data gap' for any country depends essentially, therefore, on the planning methodology used and the planning targets set. If, for example, a simple methodology were considered adequate, existing data might also be considered sufficient, whereas a 'data gap' would arise should more complex and sophisticated planning methods be contemplated.
In half the countries of sub-Saharan Africa, there are no figures more recent than 20 years old for death rates of children under 5. For nearly half the countries of Asia, the latest statistics are seven years old; in Brazil and Mexico, ten years old. Projections are often based on fragmentary and outdated surveys, at times even extrapolated from neighbouring countries. This is then used to guide social development programmes.