The Distributional Shape of Unemployment Duration: A Reconsideration
重新审视了Addison和Portugal（1987）使用扩展广义伽马分布检验失业持续时间模型稳健性的研究，指出其估计结果对分布假设敏感的原因在于使用了内生标准（是否领取失业救济金）来划分样本，并揭示了数据集的局限性。
Econometric analysis of duration data raises difficult issues of statistical inference. Building on the work of Prentice (1974) and Farewell and Prentice (1977), Addison and Portugal (1987; henceforth, AP) use the extended generalized gamma (EGG) distribution to test the sensitivity of regression estimates to more restrictive distributional assumptions which frequently have been imposed by other studies. AP find that these restrictions are rejected by data from the 1984 Displaced Worker Survey (DWS) and that imposing them creates large and variable biases in the estimated effects of the independent variables. AP also find that very different distributional forms provide the best fit for two subsamples of displaced workers. Both results suggest that many findings from earlier studies using simple parametric hazard models of unemployment duration are not robust. In this comment, we show that the sensitivity of AP's estimation results to distributional assumptions is a consequence of using an endogenous criteria to stratify the estimation sample, namely, whether or not workers collect Ul. This is an especially serious problem for a sample of displaced workers, because (a) many workers report jobless spells of zero or very few weeks; and (b) most displaced workers are Ul eligible. We also identify several other limitations of the DWS data for estimating unemployment durations and perform sensitivity analysis.