Accurate estimates of mortality at advanced ages are essential to improving
forecasts of mortality and the population size of the oldest old age group. However,
estimation of hazard rates at extremely old ages poses serious challenges to researchers:
(1) The observed mortality deceleration may be at least partially an artifact of mixing
different birth cohorts with different mortality (heterogeneity effect); (2) Standard
assumptions of hazard rate estimates may be invalid when risk of death is extremely high
at old ages; (3) Ages of very old people may be exaggerated. One way of obtaining
estimates of mortality at extreme ages is to pool together international records of persons
surviving to extreme ages with subsequent efforts of strict age validation. This approach
helps researchers to resolve the third of the above-mentioned problems but does not
resolve the first two problems because of inevitable data heterogeneity when data for
people belonging to different birth cohorts and countries are pooled together.