This document discusses aggregate demand forecasting through a case study of HP's supply chain. It explains that aggregate forecasts at higher supply chain levels are more accurate than at lower levels. The case study shows how HP forecasted demand for desktop PCs with 110V and 220V power supplies separately and in total. Forecast accuracy improved when considering total demand rather than specific configurations. Key lessons are that delayed product differentiation through aggregate forecasting and demand management helps supply chain competitiveness and cost reduction.