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Probabilities of dying ¹ or death probabilities ¹ are used to study in detail the mortality of a period or of a cohort. They are the probabilities that an individual of exact age x will die before exact age x + n, and are represented by the symbol _nq_x . If n = 1, we talk about annual death probabilities ²; if n = 5, about quinquennial death probabilities ³. The instantaneous death rate ⁴, or as it is occasionally called the force of mortality ⁴, is the limit of the _nq_x value as n tends to zero. The complement to one of the probability of dying from exact age x to exact age x + n is the probability of survival ⁶ over this interval. In the preparation of population projections, we use survival ratios ⁷; they represent the probability that individuals of the same birth cohort or group of cohorts will still be alive n years later.

• 1. The probability of death between age x and x + n is defined as the ratio of deaths between ages x and x + n to the number of survivors at exact age x. It is not to be confused with the central death rate, the ratio of deaths between ages x and x + n to the mean population alive at that age. The central death rate is written _nm_x .
• 6. The probability of survival from age x to age x + n is written _np_x .

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