Should statistical forecasts be constructed by aggregating data to each level for which forecasts are required or aggregating the forecasts from the lower levels? The relevant literature suggest no general answer. In this study using actual data, forecasts aggregated form lower-level forecasts were found best.
Dunn, D. M.; Williams, William (Bill) H. Ph.D; and DeChaine, T. L., "Aggregate Versus Subaggregate Models in Local Area Forecasting" (1976). CUNY Academic Works.