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. Before the COVID-19 recession began in March 2020, no post-World War II era had come anywhere near the depth of the Great Depression, which lasted from 1929 until 1941 (which included a bull market between 1933 and 1937) and was caused by the 1929 crash of the stock market and other factors.