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Equation 3 · Part 1 · How Demography, Migration, and Urban Futures Actually Work

Symbol P_a+1,t+1

Pa+1,t+1=Pa,t Sa,t+Ma,tP_{a+1,t+1} = P_{a,t}\, S_{a,t} + M_{a,t}
Pa+1,t+1P_{a+1,t+1}

What this part means

PaP_a+1,t+1 is part of the quantity the equation computes from the expression on the right.

Its job in the formula

PaP_a+1,t+1 is part of the quantity the equation computes from the expression on the right.

The passage around this formula

The UN Population Division builds its projections using the cohort-component method : rather than extrapolating total population, it starts from the population by single year of age and sex in a base year, and ages every cohort forward one year at a time by applying age-specific and sex-specific survival probabilities (mortality), age-specific fertility rates applied to women of reproductive age (which generates the next year’s births), and net international migration assumptions applied by age [ 1 ] . Formally, for an age group a in year t : Pa+1,t+1=Pa,t Sa,t+Ma,tP_{a+1,t+1} = P_{a,t}\, S_{a,t} + M_{a,t}. where Pa,tP_{a,t} is the population in age group a at time t , Sa,tS_{a,t} is the survival rate applied to that cohort, and Ma,tM_{a,t} is net…

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Learn the underlying idea

A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.

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Sources cited in the surrounding passage

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