A-Level地理-年龄结构Age StructureRevision: 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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