The contribution of forecast uncertainty to lifespan uncertainty

Background: Period lifespan variability is sometimes interpreted as an indicator of lifespan uncertainty for individual cohort members. Is this an admissible interpretation for living cohort members, given that future death rates are not precisely known? Objective: To answer this question, we express individual lifespan uncertainty as a function of variation in ages at death in a cohort lifetable and uncertainty about future death rates. We then quantify the importance of each component to total individual lifespan variability of a birth cohort. A small contribution of the forecast variance would support the interpretation of lifespan variability as individual lifespan uncertainty. Methods: Using Human Mortality Database data for seven European countries, we decompose the total variance in the remaining lifespan at birth, age 40, and age 80 of an individual cohort member into cohort lifetable variance and forecast variance for cohorts born 1871–2019. We do so under Lee–Carter cohort forecast variance and under an empirical forecast variance measure which allows for mortality shocks. Results: Even when accounting for the possibility of future mortality shocks, the forecast variance component tends to contribute less than 10% to the total lifespan variance of an individual. Exceptions are France and the small and thus volatile population of Iceland. Conditioning on the absence of future mortality shocks, the forecast variance contributes around 1% to total lifespan variance. Contribution: We give credibility to the interpretation of period lifetable lifespan variance as individual lifespan uncertainty by showing that the lifespan variance of a cohort is mainly determined by expected lifetable variance with smaller contributions from forecast variance.