ALTAM: UDD and constant force give different fractional mortality
Annual mortality alone does not determine survival at every fractional age. Uniform distribution of deaths and constant force are additional assumptions. They agree at a full year but can disagree inside it.
Worked example or practice scenario
With annual q=.20, UDD gives half-year survival 1−.5×.20=.90. A constant force consistent with full-year survival .80 gives half-year survival .80^.5≈.89443. The discrepancy is an assumption effect, not rounding error.
Try this next
Compute half-year death probability under both assumptions, then conditional survival from time .5 to 1. Under UDD it is .80/.90. State the starting age in each probability so an unconditional and conditional quantity are not confused.
Reading sources
ActNet editorial guide · October 1, 2026 · Original illustrative examples.
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