Equation 6 · How Demography, Migration, and Urban Futures Actually Work
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the time or time index used in this relationship. Read the equation part by part below; each part has a contextual explanation and a link to its mathematical background.
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where is the population in age group a at time t , is the survival rate applied to that cohort, and is net migration entering or leaving that age group during the interval. Births are generated separately by summing age-specific fertility rates across women of childbearing age, then applying infant survival to place the new cohort at age zero the following year. Iterating this recursion one year and one cohort at a time, across every age group and both sexes, is what actually produces the projected pyramid for a future year — not a single equation fit to a historical growth curve [ 1 ] .
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