An Evaluation of Population Projection Errors for Census Tracts

Population projections are widely used in both the public and private sectors for planning, budgeting, and analysis. For these purposes, projections are often needed for small areas such as census tracts, zip code areas or traffic analysis zones. Population size, growth constraints, shifting boundaries, and data availability create special problems for small-area projections, however, and very little is known about their forecasting performance. In this article we evaluate the accuracy and bias of projections of total population and population by age group for census tracts in three counties in Florida. We use data from 1970 and 1980 and several simple extrapolation techniques to produce projections for 1990; we then compare these projections with 1990 census counts and evaluate the differences. For the total sample, we find mean absolute errors of 17%-20% for the three most accurate techniques for projecting total population and find no indication of overall bias. For individual age groups, mean absolute errors range from 20%-29%. We believe that this analysis provides valuable information for demographers, planners, marketers, and others who make extensive use of small-area projections.