Gamma Unobserved Heterogeneity and Duration Bias
研究了在多元伽马分布混合比例风险模型中，未观测异质性对持续时间偏差的影响，并用高等教育持续时间数据验证了标准结果。
Røed et al. (1999 Røed , K. , Raaum , O. , Goldstein , H. ( 1999 ). Does unemployment cause unemployment? Micro evidence from Norway . Applied Economics 31 : 1207 – 1218 .[Taylor & Francis Online], [Web of Science ®] , [Google Scholar]) demonstrate that the standard result of known negative duration bias does not necessarily hold in a two-state mixed proportional hazard (MPH) model. We show that the duration bias is still ambiguous in a MPH model with a multivariate gamma distribution. A discrete time two-state version of our MPH model is developed to analyze the duration of higher education. The estimation results show that we cannot reject the hypothesis that the two unobserved heterogeneity variables are uncorrelated. Accepting this hypothesis implies that the standard result holds in our analysis.