This paper assesses how various approaches to modelling the separation margin a¤ect the ability of the Mortensen-Pissarides job matching model to explain key facts about the aggregate labor market. Allowing for realistic time variation in the separation rate, whether exogenous or endogenous, greatly in- creases the unemployment variability generated by the model. Speci…cations with exogenous separation rates, whether constant or time-varying, fail to pro- duce realistic volatility and productivity responsiveness of the separation rate and worker ‡ows. Speci…cations with endogenous separation rates, on the other hand, succeed along these dimensions. In addition, the endogenous separation model with on-the-job search yields a realistic Beveridge curve correlation, and it performs well in accounting for the productivity responsiveness of vacancies and market tightness. When the Hagedorn-Manovskii calibration approach is used, the behavior of the job …nding rate, vacancies and market tightness be- comes more realistic, but the volume of job-to-job transitions in the on-the-job search speci…cation is essentially zero.
