Duration of unemployment represents the length of time (through the current reference week) that persons classified as unemployed had been continuously looking for work. For persons on layoff, duration of unemployment represents the number of full weeks since the end of their most recent period of employment. Thus, it is a measure of an in-progress spell of joblessness, not a completed spell. Two useful measures of the duration of unemployment are the mean and the median. Mean duration is the arithmetic average computed from single weeks of unemployment. Median duration is the midpoint of a distribution of weeks of unemployment.

Bureau of Labor Statistics’ Handbook of Methods