Modeling Individual Earnings Trajectories Using Copulas: France, 1990-2002
Résumé
We construct a dynamic model of individual earnings which is a natural extension of the
standard discrete Markov chains used in the empirical literature on earnings mobility and
we allow for both observed and unobserved heterogeneity. Using copula representations of
multidimensional densities we decompose the likelihood of individual earnings trajectories into
the product of two components: the product of marginal–or cross-sectional–densities and the
likelihood of the sequence of individual ranks in marginal distributions. Copula representations
justify the independent analyses of cross-sectional inequality and relative mobility that one
finds in the literature. We model the year-to-year dynamics of ranks using the Plackett (1965)
parametric copula family. We use discrete mixtures of such models to characterize unobserved
heterogeneity. To estimate these mixtures, we develop a sequential EM algorithm, which is
shown to be root-N consistent and asymptotically normal. The estimation algorithm is simple
to implement and fast enough to converge for bootstrapping to be a recommendable procedure
for estimating standard errors. We then apply our methodology to French Labor Force Survey
data, for 1990-2002. We find that neglecting earnings mobility, individual heterogeneity and
unemployment risk has a significant effect on the level of intertemporal earnings inequality but
affects very little their evolution.