Abstract This paper discusses assessment of period as a factor filtering joint exogenous influence on mortality change. Environmental change may have impact for selection. Statistical modeling ignoring selection and resting on aggregate mortality data tabulated by year and age does not allow distinct identification of the factors period, cohort and age since these are linearly related. Hansen (2008, 2011) proposes an explanatory hazard model incorporating heterogeneity associated with personal genetics, biological attrition, and selection at the level of individuals. Assessment of the model rests on stochastic micro simulation. The model does not claim to be fully accurate. Its relevance is justified, however, by its documented capacity to accommodate a substantial portion of the knowledge, observations and theoretical circumstances related to change of human survivorship; instigated by advances in medical technology, sanitation, artificial immunization, and prophylaxis over the past two or three centuries. The contribution of the present paper is identification of a cohort-based period factor describing deviation from stationarity as defined by period mortality on birth of a cohort; independent of distribution by age and not assuming proportional mortality. In relation to time factors based on classic methods such as standardization by age or log-linear modeling the cohort-based period factor enables improved approach of model-based to empirical cohort mortality in epochs of significant mortality change, for example associated with long-term transition or short-term disturbances such as epidemic crises. Illustrations are provided for elected historical cohorts born in Sweden, Denmark and Iceland during the modern demographic transition. To what extent does historical survivor experience support future cohort survivorship? And what are the likely impacts for heterogeneity and expected longevity? To illustrate this issue we briefly look into expected future mortality as currently anticipated by Statistics Sweden and Statistics Denmark.