Ridder and Woutersen (2003) have shown that under a weak condition on the baseline
hazard there exist root-N consistent estimators of the parameters in a Semi-Parametric Mixed Proportional Hazard Model with a parametric baseline hazard and unspecified distribution of the unobserved heterogeneity. We extend the Linear Rank Estimator (LRE) of Tsiatis (1990) and Robins and Tsiatis (1991) to this class of models. The optimal LRE is a two-step estimator. We propose a simple first-step estimator that is close to optimal if there is no unobserved heterogeneity. The efficiency gain associated with the optimal LRE increases with the degree of unobserved heterogeneity.
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