Most nonparametric identification results for the mixed proportional
hazards model for single spell duration data depend crucially on the
proportional hazards assumption. Here, it is shown that variation in
covariates over time, combined with variation across observations, is
sufficient to ensure identification without the proportional hazards
assumption. The required variation over time is minimal, and the mixed
hazards model is identified without the proportional hazards assumption in
the presence of standard time-varying covariates.Thanks to Kåre Bævre, Zhiyang Jia, Tore Schweder,
Rolf Aaberge, and John K. Dagsvik for useful comments.