General development, structure, and uses of a computerized impact projection model, the North Dakota Regional Environmental Assessment Program (REAP) Economic-Demographic Assessment Model, were studied not only to describe a model developed to meet informational needs of local decision makers (especially in a rural area undergoing development), but also to assess elements essential to its development. The REAP model was designed to provide projections of economic, demographic, and fiscal effects of development at state, regional, county, and municipal levels for a 15-county area in western North Dakota where energy impacts were expected to be most important. The model includes variables such as business activity, personal income, employment, population by age and sex, school enrollments, housing requirements, public sector costs and revenues, and net fiscal balance. During 10 months in 1977, 45 different entities used the model for various aspects of planning and decision making. In general, the model was assessed as widely adaptable. Its development process points out factors essential to modelling efforts: models must address real informational needs, take advantage of local research expertise, be multidisciplinary, and be developed in an atmosphere of support from both state and university with a close working partnership between model developers and local data users. (RS)