Interpreting demographic effects in duration analyses of first birth intervals

"Estimated demographic effects in proportional hazard models of first birth intervals could reflect time-invariant differences in the risk of a birth, or differences in the timing of a shift in the risk, or both. This paper attempts to distinguish between these possibilities. The procedure is to estimate a more general model than the proportional hazard specification, in which the evolution of the risk of a birth can differ with demographic characteristics. The proportional hazard specification is nested within this more general model. Consequently, the consistency of the data with the 'risk' or the 'timing' interpretation of demographic effects can be tested. The data studied do not lead to a rejection of the proportional hazard specification." Data are from the 1984 U.S. National Longitudinal Survey Youth Cohort and concern 670 women aged 16 or 17 in 1979.