7.6 Mean population - average for two points of time

When demographic key ratios are calculated, population changes in a specific time period are compared with the population subject to the population change event. Because population events - births and deaths - occur throughout the year, the problem in calculation of key ratios is the risk population with which the population changes are compared. Is the risk population going to be the population at the beginning or end of the year or some population between the two?

There is a very simple solution to this problem: the average of populations at the beginning and end of the reference period is used as the population. In demography this average is called mean population.

Usually demographic key ratios are counted for the calendar year, but the reference period can also be longer. The mean population for several years is calculated so that the mean population is counted for all the years examined and the sum of these mean populations is divided by the number of the years.

When key ratios are calculated according to age and gender, the mean population is counted in the same way as described above for annual mean populations. For example, we can take 18-year-olds at the beginning of the year and 18-year-olds at the end of the year and then calculate the mean population (i.e. average) for them.

The mean population is also used for other calculations than demographic key ratios. Country comparisons can be made, for example, on consumption of coffee, number of cars and gross domestic product per 1,000 mean population.

Examples

Next lesson: Measurement of population phenomena