The rapid fall in unemployment after the Great Depression suggests that there is nothing inherently persistent in a high unemployment rate. But a closer examination of the data indicates that changes in the unemployment rate might not have been as pronounced as generally believed.
Romer, Christina, "Spurious Volatility in Historical Unemployment Data," Journal of Political Economy, February 1986, vol. 94, no. 1, pp. 1–37. (A working paper version is available online.)