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雅思阅读
2016-08-26 13:59
来源：新东方整理
作者：上海新东方
Revision: Age Structure
There are 3 basic age structures:
1. Progressive (Brazilian) Type
2. Stationery (American) Type
3. Regressive (European) Type
Progressive Type
It has a wide base that quickly narrows upwards to a point.
It indicates a large birth rate, but poor conditions mean people rapidly die off, hence the triangular shape of the structure. The population is increasing.
Stationery Type
The shape of this structure is more "square." All age groups are well represented.
It indicates that the birth rate is moderate, and few people die off as they get older. The population is stagnant.
Regressive Type
The structure has a fairly wide top with a bulging middle and narrow base.
The birth rate is low, hence the low base, and there are more adults than children. The population is decreasing.
The Dependency Ratio
This is the ratio between the non-working population (children and aged) and the workers (adults).
It is lowest in regressive populations and highest in progressive populations.
It is calculated using the formula (children + aged)/Adults) * 100
In developed countries, the DRs range from 50-70. In most developing countries the DRs are over 100
The Old Age Index
This is the proportion of aged to adults.
It is calculated using the formula: (aged)/(adults) * 100
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