7.1 Key ratio - indicator for size of population phenomena

The frequency of population phenomena is described in demography with various key ratios or index numbers. Calculation of key ratios mainly concerns counting the relative proportions of different population change events - births, deaths, migration, marriages contracted and divorces - in total population, for instance. The number of children born in one year can be compared with total population. The key ratios are often given either in per cent or per mille.

The permillage is often expressed as per thousand (1,000) population. For example, when examining the birth rate, it is said that in Finland 11.1 children per thousand population (inhabitants) were born in 1999, while in Sweden 10.0 children per thousand population were born in the same year.

In some cases when there are only a few population change events, key ratios may be presented per 10,000 and 100,000 population.