Why do I need to know this
In examinations you receive many different types of data such as tables with many numbers, demographic transition models as mentioned before and another is population pyramids.
Because of the high birth rates the bottom age group of the graph is full with a large percentage of living people there while higher up it tapers off. This is due to the high mortality rate from birth upwards and also due to the short life expectancy in these countries.
The simplest way to describe is calling it a pyramid. The reason for the shape is that there is still a high birth rate, however the death rate is going down such that more people are reaching old age or al least that life expectancy has slightly increased.
The graph begins to bulge at the middle age groups because the birth rates have gone down and more people are surviving to the middle age group and the death rate is fairly low
Stage four would be a convex graph shape because there are about as many deaths as births and more people are reaching middle age as mentioned in stage 3 but here the population is stable.
Possible stage five?
From what I have been writing here I deduce that this graph which is going into a negative growth rate would be represented by a graph which has more people in old age than young therefore giving a small base and a large bulge near the old age groups.
By looking at this we then come onto a new topic which is dependency ratios.