On Mon, 25 Apr 2011, Qi Shi wrote: Because durations are inherently non-negative. The normal distribution is not applicable. It doesn't. Duration models are just one particular sub-class of limited dependent variable (LDV) models: the dependent variable is limited in that it cannot be negative, and it may be right-censored (if some "spells" are not finished yet). Other sorts of LDV models are handled by different estimators: logit, probit (binary, ordered or bivariate), tobit, intreg (interval regression), poisson and negbin for count data, and so on. Allin Cottrell