The term "rate" is ubiquitous in demography, and often improperly used. Strictly speaking a rate relates a number of events (such as births, deaths, migrations) in its numerator, to a number of "person-years of exposure to risk" experienced by a population during a certain time period in its denominator. At the risk of over-simplifying, we can say that a rate measures the speed at which events occur.
There are several important points in the above definition of a rate:
- First, a rate refers to a given period of time that has to be specified, not just to a number of people. Rates are frequently calculated for a period of one year (annual rates), but they may also refer to shorter periods (e.g., daily or monthly rates) or longer periods (e.g., 10 years, a century).
- Second, rates in demography are calculated per "person-years", rather than simply per year or per person. Why is that necessary? In many situations the amount of time at risk of experiencing an event (e.g., giving birth) differs between members of a population. Think of the US population: each year a certain number of people die, or move out of the country. Women of reproductive age who die of an accident in July are not exposed to the risk of giving birth for the same duration as other women who have remained alive during a given year. The former are exposed to the risk of giving birth for 6 months, whereas the latter are exposed to the risk of giving birth for 1 year. We say they do not contribute as many person-years of exposure to the denominator.
- Third, "person-years" needs to be qualified. We add "of exposure to risk", and by this we aim to limit who is included in the calculation of a rate to those who may experience an event. For example, women in their 70's and 80's never bear children, so they are not at risk of giving birth. As a result, they are excluded from the calculation of fertility rates: fertility rates are frequently limited to women aged 10-49, or 15-49.
Many measures often referred to as "rates" in demography and elsewhere however are not strictly speaking "rates". For example, the crude birth rate ( = # of births / total population ) is really a ratio since it includes in its denominator males, children and the very old: these groups are not at risk of giving birth.
Some of the key measures in demography are listed below:
The sex ratio is the ratio of males to females in the population (normalized to 100). We calculate two sex-ratios: at birth, and in the total population.
The sex ratio at birth is fairly standard, around 105 men for 100 women. Due to higher mortality among males, the sex ratio in the total population switches to 95-97. For populations with high levels of sex-selective outmigration (such as male soldiers leaving a country for war), particularly in certain age groups (e.g. aged 15-29), the sex ratio may be even smaller.
This ratio quantifies the number of persons in a population who are not economically active for every 100 economically active persons in that population. It can be calculated by dividing the population 0-14 years and 65 years and older by the population that is in the 15-64 year age group.
You can calculate separate dependency ratios: child dependency ratio (Pop 0-14 / Pop 15-64) and old age dependency ratio (Pop 65+/Pop 15-64).
Ranges for child dependency ratios
- Least Developed: 80.8
- Developing: 53.1
- More Developed: 29.4
Maternal mortality ratio
The maternal mortality ratio (MMR) is the number of maternal deaths per 100,000 live births. The numerator only includes deaths to women during their pregnancy or in the first 6 weeks after delivery.
Ranges for the MMR
- Least Developed: 1000
- Developing: 440
- More Developed: 12
Crude Birth Rate (CBR)
Number of births per 1,000 persons in a population over a given period of time (i.e. 1 year).
Example: In the town of Kolikouro, Mali, there were 5663 births. The total population was 149,442. The CBR is: 5663/149,442 * 1000 = 37.9
Ranges of CBR
- Least Developed: 40.4
- Developing: 23.1
- More Developed: 11.2
Crude Death Rate (CDR)
Number of deaths per 1,000 persons in a population over a given period of time (i.e. 1 year). CDR is calculated in the same way as for CBR, but with deaths instead of births as the numerator.
Ranges of CDR
- Least Developed: 14.9
- Developing: 7.8
- More Developed: 10.2
Age Specific Death Rates (ASDRs)
In describing a population phenomenon like mortality, rates are often calculated for specific age groups of the population to gain a more sophisticated picture of how the population is changing over time. This is especially important for gauging the efficacy of health interventions that are targeted at specific segments of the population such as children or the elderly. Very high death rates among children under 1 year of age may require a very different intervention than would very high death rates among adults over 70 years of age. They are usually expressed per 1000 persons.
Infant Mortality Rate (IMR)
Infant mortality is the annual number of infant deaths among infants (i.e., under the age of 1 year) divided by the mid-year population of all infants under the age of 1 year. However, because mid-year population is only available in certain countries and only for those years in which a census takes place, the most commonly used formula is as follows:
[Number deaths of infants <1 yr. in the current year] /[Number live births in the previous year] * 1,000
Example: In 1995, Niger had 548,000 births. In 1996 the country registered 74,528 deaths of children under age 1. The IMR is estimated as: (74,528/548,000) * 1000 = 136.
The IMR is the most widely used indicator of population health. Because an infant's death is reflective of the mother's well-being and nutritional status during pregnancy, the child's nutritional status after birth and the child's lack of protection against preventable, infectious diseases, a high IMR is an indirect measure of poor overall health and poor levels of living.
Range of IMR
- Least Developed: 102
- Developing: 56
- More Developed: 8
Other measures commonly used to describe health and mortality at young ages include: the neonatal mortality rate, the child mortality rate, and the under-5 mortality rate.
Age Specific Fertility Rate (ASFR)
Age-specific fertility rates are the total births in the calendar year to all women of the designated age group number of births to mothers of each age (or age group) by the number of women that age (or age group) in the population. This rate is usually expressed as births per 1,000 women of that age. ASFR can only be calculated with data on the age of the mother at birth of the child. For example the age-specific fertility rate of women aged 20-24 in 2005 can be written as:
Number of births to women aged 20-24 in 2005 / # of person-years lived by women aged 20-24 during 2005