An Example of Combining Expert Judgment and Small Area Projection Methods: Forecasting for Water District Needs

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Spatial Demography

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This case study shows how GIS and Expert Judgment can be used to develop small area population forecasts in the United States. It starts by organizing 2000 and 2010 block group population data into the 2020 block group geography and then examines 2020 indicators used to evaluate the effect of Differential Privacy on the 2020 population data. Preliminary population projections to 2050 are then generated by averaging the results of three standard small area projection methods. Using local expert judgment, GIS overlay maps and satellite imagery in a virtual environment, the 301 block groups of Greenville County, South Carolina were classified into seven categories of future population change. These categories were then applied to the preliminary projections to generate informed forecasts. Following this step, the sums of the BG results were then compared, respectively, to independently generated county population forecasts for 2030, 2040, and 2050. At this point, 25 BGs were selected for additional review, which resulted in a final set of forecasts. We find that the increase of 152,840 people in the year 2050 spread over all of the 301 census block groups in going from the preliminary projections (675,626) to the final informed forecasts (828,467) is largely generated by these same 25 BGs, which expert judgment determined were currently poised to “take off” in terms of population growth. Having this much change generated by such a small number of BGs is consistent with findings elsewhere.


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