Final answer:
Government economists at the Bureau of Economic Analysis use various sources to measure GDP and produce estimates on a quarterly basis. They cross-check information on international flows of goods and capital and revise these estimates regularly.
Step-by-step explanation:
Government economists at the Bureau of Economic Analysis (BEA) use a variety of sources to measure GDP. They piece together estimates of GDP by cross-checking information on international flows of goods and capital against other available data. The BEA then melds all this information together to produce GDP estimates on a quarterly basis, which are later revised and updated as more information comes in. The BEA releases advance, preliminary, and final estimates for each quarter, and also revises past GDP estimates every five years based on the newest methods and data.