Equation 2 · Part 1 · How Demography, Migration, and Urban Futures Actually Work
Symbol t
What this part means
the time or time index used in this relationship.
Its job in the formula
t is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.
Full expression→Symbol t→Article meaning
Where the article explains it
Formally, for an age group a in year t : = \, + 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.
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 :
Learn the underlying idea
A variable is a named place for a value. Its letter is a local label: x can mean position in one formula and a data point in another.
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Sources cited in the surrounding passage
These citations provide research context; check each source for the exact claim it supports.