<P> The unofficial beginning and ending dates of recessions in the United States have been defined by the National Bureau of Economic Research (NBER), an American private nonprofit research organization . The NBER defines a recession as "a significant decline in economic activity spread across the economy, lasting more than two quarters which is 6 months, normally visible in real gross domestic product (GDP), real income, employment, industrial production, and wholesale - retail sales". </P> <P> In the 19th century, recessions frequently coincided with financial crises . Determining the occurrence of pre-20th - century recessions is more difficult due to the dearth of economic statistics, so scholars rely on historical accounts of economic activity, such as contemporary newspapers or business ledgers . Although the NBER does not date recessions before 1857, economists customarily extrapolate dates of U.S. recessions back to 1790 from business annals based on various contemporary descriptions . Their work is aided by historical patterns, in that recessions often follow external shocks to the economic system such as wars and variations in the weather affecting agriculture, as well as banking crises . </P> <P> Major modern economic statistics, such as unemployment and GDP, were not compiled on a regular and standardized basis until after World War II . The average duration of the 11 recessions between 1945 and 2001 is 10 months, compared to 18 months for recessions between 1919 and 1945, and 22 months for recessions from 1854 to 1919 . Because of the great changes in the economy over the centuries, it is difficult to compare the severity of modern recessions to early recessions . No recession of the post-World War II era has come anywhere near the depth of the Great Depression, which lasted from 1929 until 1941 and was caused by the 1929 crash of the stock market and other factors . </P> <P> Attempts have been made to date recessions in America beginning in 1790 . These periods of recession were not identified until the 1920s . To construct the dates, researchers studied business annals during the period and constructed time series of the data . The earliest recessions for which there is the most certainty are those that coincide with major financial crises . </P>

When was the longest recorded recession in us economic history