The importance of consecutive spells of poverty: a path-dependent index of longitudinal poverty

In this paper we propose a new index of individual poverty in the longitudinal perspective, taking into account the way poverty and non-poverty spells follow one another along individual life courses. The Poverty Persistence Index (PPI) is based on all the pairwise distances between the waves of poverty. The PPI is normalized and it assigns a higher degree of (longitudinal) poverty to people who experience poverty in consecutive, rather than separated, periods, for whom the distances from the poverty line are larger along time and moreover, when the worst years are consecutive and/or recent. We also propose an aggregate index of persistence in poverty (APPI) in order to measure the distribution of the persistence of poverty in a society, and evaluate at once the diffusion of poverty, its depth, duration, and recentness. The indices are tested in comparison with other measures from the literature both at the individual as well as at the societal level.