In a life table, dx is calculated as:
“nx - nx+1” for in a life table, dx is calculated as. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. Selection favors the schedule that increases lifetime reproductive success under local mortality and resource conditions. Body size, development time, fecundity, parental investment, and generation length consequently tend to covary. The remaining alternatives—“nx/n₀”, “dx/nx”, “lx * mx”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Life
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 10
