C OMMON C ONCEPTS AND I NDICATORS Most demographic concepts are expressed as rates or ratios – they involve two numbers. One of these numbers is the particular statistic that has been calculated for a specific geographical-administrative unit; the other number provides a standard for comparison. For example, the birth rate is the total number of live births in a particular area (an entire country, a state, a district or other territorial unit) during a specified period (usually a year) divided by the total population of that area in thousands. In other words, the birth rate is the number of live births per population.
The death rate is a similar statistic, expressed as the number of deaths in a given area during a given time per population. These statistics depend on the reporting of births and deaths by the families in which they occur. The rate of natural increase or the growth rate of population refers to the difference between the birth rate and the death rate. When this difference is zero (or, in practice, very small) then we say that the population has ‘stabilised’, or has reached the ‘replacement level’, which is the rate of growth required for new generations to replace the older ones that are dying out.
Sometimes, societies can experience a negative growth rate, that is, their fertility levels are