Age structure.< /s> The distribution of a population accor ding to age, usually by 5-year age grou ps. Age-specific fertility rate. The n umber of births dur ing a year to women in a particular age group, usually per 1 ,000 women in a 5-year age group at midy ear. Aging. An increase in the < lex pos=NN>proportion of the population in the older ages. May also be measured as an inc rease in the median< /lex> age of the population. AIDS. Acquired immune deficiency syndrome. Base population. The population,< /lex> usually by age and sex, for the initial y ear of a projection< /lex>. Birth rate. The average annual number of births during a year per 1,000 population at midyear. Also known as the crude birth r ate. Children ever born. The total number of births a woman has had, regardless of whether the children are living or dead at the time of the inquiry. Children surviving. The number o f children a woman< /lex> has had that are still living at the time of the inquiry . Cohort. A group of individuals born in the same calend ar year or group of years. Cohort component metho d. See compo nent method. Component method. A method of estimating or projecti ng a population in which separate comp onents of population change (fertility , mortality, and migration) are used to derive the total population. When such projections are made also by age and sex, the< /lex> procedure is known as the cohort component method.< /s> Components of change< /lex>. Fertility, mortality, and migration. Contraception. The conscious effort of couples to regulate the number and spacing of births. Also known as family planning. Contraceptive prevalence rate. The percent of currently married women of reproductive age (n ormally defined as the range 15 to 49 years) who use contraception. Crude birth rate . See birth rate. Crude death rate . See death rate. Currently married wom en. Women ages 15 to 49 either formally married or living in union with a man (consensual unions). S ame as "married women of reproductive age." Death rate. The average annual number of deaths during a year per 1,000 population at midyear. Also known as the crude death r ate. Development category. The classification of regions into "less developed" and "more< /lex> developed" accord ing to their general level of economic development. In this report, countries are classified according to the grouping used by the United Nations.< /lex> See references to these terms i n the Gloss ary for details. DHS . Demographic and Health Surveys, an ongoing program of household surveys implemented by< /lex> Macro Inte rnational, Inc. and collaborating organizatio ns. Family planning. See contraception.Growth rate. The average annual percent change in the population, resulting from a surplus (or deficit) of births over deaths and the balance of migrants entering and leaving a country. The rate may be positi ve or negative. Also known as population growth rat e or average annual< /lex> rate of growth . HIV . Human immunodeficiency virus. The < lex pos=NN>virus that causes AIDS. Indirect estimation.< /lex> The use of special techniques to estimate demographic measures (such a s fertility and mort ality) when inform ation is not adequa te for measuring t hem directly. Infant mortality rate< /lex>. The number< /lex> of deaths of infants under 1 year of age from a cohort of 1,000 live births. Denoted 1 q 0 or IMR, it is the p robability of dying between birth and e xact age 1. IUD . Intrauterine device, a method of contraception. iwgAIDS. Interagency Working Group on AIDS. Less developed coun tries. The "less developed " countries include all of Africa , all of Asia ex cept Japan, the Transcaucasian and Central Asian republics of the NIS, all of La tin America and the Caribbean, and all of Oceania except Australia and N ew Zealand. This category matches the "less developed country" classification employed by the United Nations. "Less developed" countries are also referred to in the report as< /lex> "developing" countries. Life expectancy at birth. The average number of years a group of people born in the same year can be expected to live if mortality at each age remains constant in the future. Life table. A statistical table that follows a hypothetical cohort of 100,000 persons born at the same < lex pos=NN>time as they progress through successive ages, with the cohort reduced from one age t o the next accordin g to a set of death rates by age until al l persons eventually die. Married women of reproductive age (MWRA)< /lex>. Women age s 15 to 49 either formally married or living in union with a man (consensual un ions). Same as "currently married women.< lex pos="''">" Median age. The midpoint age that separates the younger half of < lex pos=DT>a population from the older half. Modern methods of contraception. Condoms, injectables, < lex pos=NNP>IUD's, pills, vaginal methods (spermicides, diaphragms, or caps),< /lex> and voluntary steril ization of a woman or her partner. More developed coun tries. The "more developed" countries and are as include all of North America and Eur ope (including the Baltics and the four European republics of the NIS) p lus Japan, Australia, and New Zealand. This category matches the "more developed" classification employed by the United Nations . Natural increase. The difference b etween the number of births and the number of deaths . Net migration rate. The difference between the number< /lex> of migrants enteri ng and those leaving a country in a year, per 1,000 midyear p opulation. May also be expressed in percent. A positive figure is known as a net immigration rate and a negative figure as a net emigration rate. New Independ ent States (NIS). Fifteen nations formed from the former Soviet Union. The Commonwealth of Independent States (CIS) refer s to these countrie s excluding the thr ee Baltic nations o f Latvia, Estonia, and Li thuania. Pandemic. A global epidemic. Projections. Data on population and vital rates de rived for future ye ars based on stati stics from population censuses, vital registration systems, or sample surveys pertaining to the recent past, and< /lex> on assumptions abou t future trends. Rate of natural increase. The difference between the crude birth rate and the crude death rate. Replacement level fert ility. The a verage number of ch ildren each woman wo uld have to bear for a population to remain the same size over the long term. Conventionally taken to be an avera ge of 2.1 children< /lex> per woman. Seroprevalence. The percent of a population testing positive for infection in a blood test. In the context o f this report, the percent testing positive for antibodies to HIV. Sustainable development. The term refers to achieving economic and social development in ways that do not exh aust a country's natural resources. See, also, Ashford (1995) and The World Commission on Environment and Development (1987). In the Commission's words. "... sustainable development is ... a process of cha nge in which the exploitation of resou rces, the direction of investments, the orientation of technological development, and institutional change are made consistent with the future as well as< /lex> present needs" (Ibid< /ENAMEX>. 9). Total fertility rate. The average< /lex> number of children< /lex> that would be born per woman if all women lived to the end of their childb earing years and b ore children according to a given set of age-specific fer tility rates. Traditional methods o f contraception. Periodic abstinence, rhythm, withdrawal, douche, an d folk methods. Also known as< /lex> natural methods. Under-5 mortality. Number < lex pos=IN>of deaths of children under 5 years of age fro m a cohort of 1,000 live births< lex pos=".">. Denoted 5q0 , it is the probability of dying between birth and exact age 5. Underenumeration . In a census, the erroneous counting of fewer persons in a population than a ctually belong to i t. Underregistration. In a vital registration system, the failure to register all vital events that occur in a popu lation. Unmet need for < lex pos=NN>family planning. Nonuse of contracepti on among women who< /lex> would like to regulate their fertili ty, measured as the proportion of currently married women of reproductive age not using contrace ption but wishing e ither to postpone th e next wanted birth or to prevent unwanted childbearing aft er having achieved their desired number of children. Vital events.< /s> Births and deaths . Vital rates. Birth rates and death rates. Vital registration. The recording of vital events for legal, administrat ive, and statistica l purposes. WHO. World Health Organ ization. WHO/GPA. World Health Organization/ Global Programme on AIDS.