to receive a percentage. When this statistic is achieved, the population growth between two or more nations that differ in size, can be accurately measured and examined. Standardization of population numbers For there to be a significant comparison, numbers must be altered for the size of the population that is under study. For example, the fertility rate is calculated as the ratio of the number of births to women of childbearing age to the total number of women in this age range. If these adjustments were not made, we would not know if a nation with a higher rate of births or deaths has a population with more women of childbearing age or more births per eligible woman. Within the category of standardization, there are two major approaches: direct standardization and indirect standardization. Common rates and ratios The crude birth rate, the annual number of live births per 1,000 people. The general fertility rate, the annual number of live births per 1,000 women of childbearing age (often taken to be from 15 to 49 years old, but sometimes from 15 to 44). The age-specific fertility rates, the annual number of live births per 1,000 women in particular age groups (usually age 15–19, 20–24 etc.) The crude death rate, the annual number of deaths per 1,000 people. The infant mortality rate, the annual number of deaths of children less than 1 year old per 1,000 live births. The expectation of life (or life expectancy), the number of years that an individual at a given age could expect to live at present mortality levels. The total fertility rate, the number of live births per woman completing her reproductive life, if her childbearing at each age reflected current age-specific fertility rates. The replacement level fertility, the average number of children women must have in order to replace the population for the next generation. For example, the replacement level fertility in the US is 2.11. The gross reproduction rate, the number of daughters who would be born to a woman completing her reproductive life at current age-specific fertility rates. The net reproduction ratio is the expected number of daughters, per newborn prospective mother, who may or may not survive to and through the ages of childbearing. A stable population, one that has had constant crude birth and death rates for such a long period of time that the percentage of people in every age