Demographic events are distributed over time, their intensity changes with the age of people, and this generally banal truth is very important in developing the principles and methods of demographic analysis. The bulk of births occur to women aged 15 to 49 years, that is, to the contingent of the so-called childbearing, or reproductive, age. This means that the more women of a given age in the population, the more, other things being equal, the number of births can be. The intensity of mortality by age has practically two «peaks»: at the age of up to one year, when the child’s body is most defenseless, and at older ages. There are many such examples, but the conclusion is clear even now — it is necessary to study demographic processes in close connection with the age composition of the population..
The number of demographic events and their structure are constantly changing over time. So, if in 1973, of the total number of births in the country, the share of second births in the order of birth accounted for 27.6%, and the share of third births — 10.0%, then in 1983 second births accounted for 35.4% of their total number. , and the third — 11.5%. This means that it is necessary to use such methods of demographic analysis that would make it possible to reveal the detailed features of the processes taking place in the population, their structural characteristics..
This implies another important requirement for the methods of demographic analysis — the need to compare them in dynamics, to compare characteristics from year to year, one period with another, in order to identify general trends of change, general patterns of ongoing processes.