DS-20 · Small-area population forecasting

Forecast tract-level population five years out for a Richmond-area county. Then do the real work: quantify how wrong such forecasts typically are, by backtesting against tracts you already know the answer for.

Census ACS 5-year time series (https://www.census.gov/data/developers/data-sets/acs-5year.html). Census building permits (https://www.census.gov/construction/bps/index.html).

ACS 5-year estimates are overlapping rolling averages — treating consecutive vintages as independent annual observations badly understates volatility. Tract boundaries change between decennial censuses. Margins of error on small tracts are large and usually ignored.

A backtested forecast with prediction intervals that account for ACS margins of error, boundary changes handled explicitly, and a plain statement of the tract sizes at which forecasting stops being meaningful.

CT: understanding overlapping-sample structure. CR: designing a backtest with limited history. CO: comparing interval calibration with a peer. CM: communicating uncertainty to a planner. CZ: population forecasts drive school siting and service allocation.

A demographer, regional planner, or statistician reviews the backtest design at Checkpoint 1 and interval calibration at Checkpoint 2. School-supervised.

R1: forecast horizon and geography fixed. R2: boundary changes handled. R3: a naive persistence or linear-trend baseline. R4: intervals are calibrated, not decorative. R5: a planner could act on it. R6: states the minimum tract size for a credible forecast.

(a) Overlapping ACS vintages are treated as independent years and the intervals come out absurdly tight. (b) A point forecast is delivered with no interval, for tracts whose margin of error exceeds the predicted change.