In the previous chapters considerable emphasis has been given to the duration of an unemployment spell. In chapter one we saw its significance in the measurement of the costs of unemployment. In chapter two it was clear that the re-employment chances of the unemployed, which determine the time spent out of work, are crucial in explaining both cross section differences in the incidence of unemployment and the secular increase in unemployment in Britain. Indeed the duration of a spell of unemployment can be measured as the reciprocal of the re-employment or outflow probability (θt) for unemployed workers.