# Why population / Life Expectancy?

Market sizing
New answer on Jul 12, 2021
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This question is about what Fernando is refering to as "Replacement Concept" in estimating the number of babies born per year:

I get that he is assuming that the # of people born = number of people dead per year to keep the population balanced at 7B according to his assumption but then he assumes that the number of people dead per year = 7B (population) / life expectancy (70 yo). Why is that?

I am trying to tie it to how we calculate the % of people falling into an age group following a uniform distribution: % of people between 15 yo and 45 yo with a life expectancy of 70 yo is (45-15)/70 = 3/7 and then *100 we get the %.

OR

does it stem from the concept of replacing the 7B every 70 years so what would be the number of people being replaced (the dead are replaced by the born) per year and that is 7B / 70 years ?

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Hi there,

If life expectancy is 70 years, then what're the odds of dying in a given year? The answer is 1/70...

So...any given person has a 1 in 70 chance of dying in a given year. Therefore, if each person in a population of 7B has a 1/70 chance of dying, then 7B/70 = how many people will die in that year!