Wednesday, 5 May 2010

Population rates

There are three fundamental mechanisms which influence populations and their structure: births, deaths and migrations. The purpose of this is to introduce some of the basic calculations which are used to understand the nature of populations. Typically, demographic variables are expressed as rates. A rate is a measure that reflects the frequency of an event (such as birth), relative to the population that may experience that event. Rates are useful because they allow us to make comparisons between different populations and because they can be compared across time to discover trends in a particular population.

Measuring Fertility: CRUDE BIRTH RATE

The crude birth rate (often referred to simply as the birth rate) is the most commonly used index of fertility. This is the ratio of the number of live births each year to the total population (usually measured at the mid-point of the year). It is expressed as the number of births per 1,000 population. e.g. if 3,000 babies were born in a population of 150,000, then the crude birth rate would be 20 per 1,000.

1. Why is it called a crude rate?

Measuring Mortality: CRUDE DEATH RATE

The crude death rate is the number of deaths per 1,000 members of a given population. It may be calculated for the population at the mid-point of the year or at the beginning of the year.

Population Growth: NATURAL INCREASE

Natural increase is a simple measure of population growth which examines the differences between births (fertility) and deaths (mortality) in a given group. It is usually determined by subtracting the crude death rate from the crude birth rate. Natural increase is generally expressed as a percentage figure. e.g. an annual natural increase of 0.8 means that a country is increasing its population by 0.8 per cent each year.

If the death rate is greater than the birth rate, then a population may be experiencing natural decrease.

2. What aspect of population growth or decline is not measured by the natural increase calculation?

Births and Deaths in the Republic of Ireland, 1995 – 2002 (selected years)

Source: CSO, 2004















Total Population (estimated)





3. Calculate the Birth and Death Rates for Ireland in each of the four years

4. Calculate the Natural Increase for Ireland in each of the four years.

5. Write a short paragraph outlining the population changes experienced over the period from 1995 to 2002, based on this data.

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