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Handout for Part 1
Introduction to Population Projections
Slides^
Slide Content
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Introduction to DemProjan e-learning course
This course is the first e-learning course created by theUnited States Agency for International Development HealthPolicy Initiative. My name is John Ross, and I will be yourinstructor for Part 1. This course was created in response tothe demand for technical assistance for use of the suite ofmodels known as Spectrum. The Spectrum software wascreated by predecessor projects with funding from USAID.The Spectrum models are used as planning tools around theworld by policymakers, program managers, students, andpeople working in the field of public health. This coursefocuses on the first module of Spectrum—DemProj—which,as its name implies, is used to make population projectionsbased on demographics.
Overview of Course Part 1: Introduction to population projections Part 2: Introduction to the DemProj modelPart 3: Programmatic applications ofDemProj
This course is divided into 3 parts. Part 1 is an introduction topopulation projections. Part 2 is an introduction to theDemProj Model. Part 3 presents some programmaticapplications of DemProj. To begin this course, users shouldhave a basic understanding of Windows software. Spectrumis a Windows-based program that will run in either Windows95 or higher versions. The Spectrum program requiresapproximately 16MB of hard disk space. The tutorials that weare about to cover can be installed from a CD or over theweb.
Introduction to Population Projections
The first part of the course is an introduction topopulation projections.
Objectives The objectives of part 1 of the course are asfollows: ^ Introduce basic concepts related to populationprojections including fertility, mortality, andmigration ^ Explain how population projections can beuseful decision-making tools for policy makers ^ Describe principal determinants of populationgrowth ^ Provide an overview of the impact of AIDS onpopulation growth
The objectives of Part 1 are as follows: Introduce basicconcepts related to population projections, includingfertility, mortality, and migration. Explain howpopulation projections can be useful decisionmakingtools for policymakers. Describe principal determinantsof population growth. Provide an overview of theimpact of AIDS on population growth.
1. Population Projections
The next few slides give an overview of populationprojections.
is for a population with a high birth rate, a high deathrate, and a short life expectancy. This is the typicalpattern for less economically developed countries.
We can also estimate what that population will looklike in the future.
When we make a population projection, we canestimate what a population will look like at some timein the future and can make comparisons between thepyramids. Notice how the bottom few bands of thepopulation in 2025 are relatively equal. This is due toassumptions about reductions in both the infantmortality rate and the fertility rate.
Population Projection ^ A “best-guess” calculation of the number ofpeople expected to be alive at a future date,based on assumptions about population size,births, deaths and migration. ^ Always based on a ‘conditional’ future.
A population projection is defined as a “best-guess”calculation of the number of people expected to bealive at a future date, based on what we know aboutthe current population size and what we expect tohappen to births, deaths, and migration. Populationprojections are always set on a “conditional” futurebecause we can never be certain about theassumptions we use in the projection. We canhowever involve multiple stakeholders and experts inworkshops when we make these assumptions.
Why make population projections? ^ Planning^ –^ Assessing the need for new jobs, teachers,schools, doctors, nurses, urban housing,food, etc. ^ Policy dialogue^ –^ Helping policymakers understand thatproblems exist^ –^ Developing solutions
Why do we make population projections? Populationprojections are useful for a number of reasons andhelp stakeholders plan for the near and distant future.If we know how many people are in a country orregion, this puts us in a better position to assess theneed for new jobs, teachers, schools, doctors, nurses,urban housing, food, and requirements for resources.For example, in order to plan an immunization programat some time in the future, governments, donors, andhealthcare staff need to know how many children willbe alive in the future. Population projections can helpus estimate future population size. Populationprojections are also important for raising awareness ofissues among policymakers. For example, apopulation projection can help illustrate the impact ofan increased population on the use of fuel wood andthe potential threat to the forests or the need foraffordable housing projects to accommodate the largeand growing population.
Summary of Key Population Concepts ^ A^ population
is the total number of men, women, boys, and girls, of different ages, livingin a particular place at a particular point in time. A^ population pyramid
is a graphic representation of the age and sex distribution. A^ population projection
is an estimate of the
number of people expected to be alive at afuture date, based on assumptions ofpopulation size, births, deaths, and migration. Population projections are useful tools for program
planning
and^ policy
dialogue.
In summary, a population is the total number of men,women, boys, and girls of different ages, living in aparticular place at a particular point in time. Apopulation pyramid is a graphic representation of theage and sex distribution. A population projection is anestimate of the number of people expected to be aliveat a future date, based on assumptions of populationsize, births, deaths, and migration. Populationprojections are useful tools for program planning andpolicy dialogue.
Reference Bureau’s World Population Data Sheet, andthe World Bank’s World Development Indicators.
Fertility ^ The biological component of fertility is thephysical ability of a woman to reproduce( fecundity
). Over the course of her life, a woman could bear between 13 and 17 children,in the absence of any other factors. The number of children born to a woman variesby social factors that affect when she startschildbearing, the spacing between children, andwhen she stops childbearing.
The biological component of fertility is the physicalability of a woman to become pregnant and carry thepregnancy to a live birth; this is also called fecundity.Over the course of her life, a woman could bearbetween 13 and 17 children, in the absence of anyother factors. The biological ability to bear children issimilar across different societies; although, it is alsoaffected by health factors such as nutrition anddisease. The number of children a woman will havedepends not only on her fecundity but also on socialfactors that affect when she starts childbearing, thespacing between children, and when she stopschildbearing. For example, families in agriculturalsocieties often have more children than families inindustrialized areas.
Measures of Fertility Crude birth rate (CBR): The number of live birthsper 1,000 persons in a given year CBR =^ (
births in a year
)^ X 1000 Mid-year population
The crude birth rate is the number of live births per1,000 population in a given year. This measure tells ushow many children will be born in a given population ina given year. It does not tell us who is having children,or how many children the typical woman might have.
Measures of Fertility^ ^ Age-specific fertility rate (ASFR):
The
number of births occurring annually per 1,000women of a specific age (usually given in 5-year age groups)
ASFR is the number of births occurring annually per1,000 women of a specified age, usually given in 5-year age groups. ASFR is the number of birthsoccurring annually per 1,000 women of a specific age,usually given in 5-year age groups. It is computed bydividing the number of children born to mothers in agiven age group by the total number of women in thatage group, multiplied by 1,000. ASFRs are moreprecise than other measures, but they also requiremore data; they require births by the age of the motherand the distribution of the population by age and sex.
ASFR for Bangladesh, 2004 Age Group
ASFR
15-^
20-^
25-^
30-^
35-^
40-^
45-^
This example from Bangladesh shows the differentfertility rates for the various 5-year age groups. Forexample, 135 live births occurred per 1,000 womenages 15–19 years old, but 192 births in the next agegroup, ages 20–24, where more women were married.Notice the definition of childbearing years orreproductive age (15–49). While some women youngerthan age 15 or older than age 49 do get pregnant andhave children, natural fecundity at these very youngand older ages is lower than during the age range of15–49. Therefore, the international convention is todefine reproductive ages as 15–49.
ASFR for Bangladesh, 2004
This graph plots the age-specific fertility rates forBangladesh. By looking at this graph, can you seewhat the age-specific fertility rate was among womenages 30–34?
Measures of Fertility^ ^ Total fertility rate (TFR):
The approximate
number of births that a woman will have if shemoves through her reproductive years havingbirths at the current age-specific birth rates. TFR is a synthesis measure of the number ofbirths women of different ages are having now.
Total fertility rate (TFR): The approximate number ofbirths that a woman will have if she goes through herreproductive years having births at the current age-specific birth rates. The TFR is a synthesis of thenumber of births women are having today.
Measures of Fertility ^ TFR is the sum of the age-specific rates(ASFRs) multiplied by 5 and divided by 1000. ^ TFR is expressed as a rate per woman. ^ TFR can be compared across populationsbecause it is not influenced by differences inage structure.
TFR is calculated as the sum of the ASFRs through allages, multiplied by 5, and then divided by 1,000. Themultiplication by 5 is because a woman will spend 5years of her life in each 5-year age group. We divideby 1,000 because the original rates were per 1,000,and we want to express the rate per individual woman.The TFR is interpreted as the number of births perindividual woman. It can be compared acrosspopulations because it is not influenced by differencesin age structure.
Calculating TFR from ASFR Calculation of TFR from ASFR: (135+192+135+83+41+16+3)*
=^ 3.
*Note that we multiply total ASFRs by 5 becauseeach age group covers five years.
Let’s use the same example from Bangladesh in 2004to calculate the TFR. We calculate the TFR bysumming all the age-specific fertility rates andmultiplying by 5, because each age group covers 5years, and then divide by 1,000. The TFR inBangladesh in 2004 was 3.0.
Comparison of TFR among Countries Country
TFR
Nigeria (2003)
Ghana (2003)
Bangladesh (2004)
Colombia (2005)
USA (2006)
United Kingdom (2006)
You will rarely have to calculate the TFR from theASFRs by hand. The TFR values above show thatsocietal factors influence the rate. The TFRs acrossthese 6 countries are markedly different. Notice thatthe first three countries, which are largely agrarian,have higher fertility rates.
Measures of Fertility ^ Replacement level fertility:
The TFR at which
women have exactly the number of birthsrequired to replace themselves and theirpartners.
Another common concept is replacement-level fertility.Replacement-level fertility is the TFR at which womenhave the number of births required to replacethemselves and their partners but no more. If nochildren died before reaching adulthood, replacement-level fertility would be 2 births per woman (1 to replacethe mother and 1 to replace her spouse). If death ratesare low, replacement-level fertility averages about2.12, since not all children will survive to reachreproductive age. If mortality rates are high,replacement-level fertility will be higher.
Sex ratio at Birth ^ Sex ratio at birth:
The number of male births per 100 female births. In most countries, this value is about 105 forfirst births.
Another measure associated with births is the sex ratioat birth. It is measured as the number of male birthsper every 100 female births. In most countries, thisvalue is 103–105, which means that for every 100 girlsborn, there are between 103 and 105 boys born. Thisis also an input in the DemProj Model.
Measures of Mortality ^ Crude death rate (CDR):
The number of
deaths in a given year, per 1000 people in thepopulation in the middle of that year. CDR = (^ number of deaths
) X 1,
mid-year population
We express the crude death rate (CDR) per 1,000persons. It is defined as the number of people who diein a given year divided by the number of people in thepopulation in the middle of that year, multiplied by1,000.
Measures of Mortality ^ Infant Mortality Rate (IMR):
The number of
deaths to children under 1 year of age per1,000 live births in the same year. IMR =^ number of deaths to^ children < 1 yr. old
X 1,
number of live births
Demographers call children under 1 year of age“infants” and calculate the mortality rate in this agegroup by dividing the number of deaths in childrenunder 1 year of age by the number of live births in aparticular year, multiplied by 1,000.
Comparison of IMR Among Countries, 2005 Country
IMR
Afghanistan
Nigeria^
Swaziland
Ghana^
Thailand
USA^
Canada
Japan^
The infant mortality rate (IMR) is considered one of themost sensitive measures of a nation’s health. In lessdeveloped countries, the chances of dying are greatestat infancy and remain high during the first few years ofchildhood. When a country has a high rate of infantdeath, it usually signals high risks from infectious,parasitic, communicable, and other diseasesassociated with poor sanitary conditions andmalnourishment. As the table shows, as countriesdevelop economically, infant mortality usually declines.
Measures of Mortality ^ Under five mortality rate (U5MR):
The
number of deaths to children under the age offive per 1,000 live births in a given year.
DemProj produces a number of mortality rates, andyou may already be aware of these. The “under fivemortality rate” is the number of deaths to childrenunder 5 years of age per 1,000 live births in the sameyear.
Measures of Mortality ^ Life expectancy at birth:
The average
number of years a new cohort of infants canexpect to live based on current mortalityconditions.
Life expectancy at birth is the average number of yearsthat a new cohort of infants can expect to live basedon current mortality conditions. It is a useful measurethat summarizes the implications of current age-specific mortality patterns on lifelong survival.
Measures of Mortality ^ Maternal mortality ratio (MMR):
The number
of women who die due to pregnancy andchildbirth complications per 100,000 live birthsin a given year.
Another measure that we will not cover in DemProj butwill be addressed in follow-on models, such as theSafe Motherhood Model, is the maternal mortality ratio(MMR). The MMR is defined as the number of womenwho die due to pregnancy and childbirth complicationsper 100,000 live births in a year. You may wonder whythis ratio is calculated per 100,000 rather than per1,000. This is because the ratio rarely exceeds twopercent of mothers dying, and it is easier to have moredigits in the rate to compare figures across countries.
4. Migration
There are other measures of a population that help usunderstand population projections. The size of apopulation is not only affected by births and deaths butis also affected by the number of people coming intoand out of a place.
Measuring Migration ^ People move different distances. ^ Some migrants are ‘return migrants.’ ^ Some migrants are not official/legal and mayview their ‘residence’ differently.
Generally, migration is more difficult to measure thanfertility and mortality because of some complexitiesnoted here. First, we need to define whether we aretrying to measure domestic or international migration.People move within the country all the time. Somemigrants are “return migrants.” For example, manymigrant workers in southern Africa migrate to SouthAfrica to work in the diamond mines on a seasonalbasis. Some migrants are not official/legal and mayview their “residence” differently. For example, thereare many people moving across the border intorefugee camps. It’s not only difficult to find data on this,but it is also difficult to define permanent residency.Nevertheless, when data are available, migration ismeasured with rates that are similar to fertility andmortality measures.
Measures of Migration ^ Net migration:
The difference between those who move in and those who move out of anarea
Net migration is the measure used as an input to apopulation projection in DemProj. It is the differencebetween those who move in and those who move outof the area for which the population projection is beingprepared. If the projection is for a country, then it isinternational migration. If the projection area is aregion or city, then migration refers to people movinginto or out of the region or city.
Measures of Migration Net migration: ^ Negative net migration implies that there aremore people moving out than in. ^ Positive net migration implies that there aremore people moving in than out. ^ Zero net migration does not necessarily meanthat nobody is moving in or out of an area.
Negative net migration implies that there are morepeople moving out. Positive net migration implies thatthere are more people moving in. Note that zero netmigration does not necessarily mean that we do nothave people moving in and out of an area in a givenyear. It might mean that equal numbers of people aremoving in and out.
Measures of Migration ^ Net international migration is not a majorcomponent of population change in mostcountries. ^ Age and sex patterns of migration varyconsiderably. ^ Net migration can be temporary or can varythroughout the year.
Net international migration is not a major component ofpopulation change in most countries. Often, migrationcan be ignored without a significant effect on thepopulation projection. However, for some countries,and also for cities, migration can be very important.Age and sex patterns of migration vary considerably.In Nairobi, for example, migrants to the city consistlargely of young males seeking work. In other cities,migrants to the city are composed primarily of entirefamilies. As noted above, net migration can also betemporary. For example, in Jordan, there was asignificant outflow of migrants during the oil boom inthe Persian Gulf states in the 1970s and 1980s.However, during the 1990s, there was a net inflow ofmigrants as families returned to Jordan due to reduced
5. Population Growth
There are other measures of a population that arenecessary to understand population projections.
Population Growth Concepts ^ Net migration:
The difference between the numbers moving in and the numbers movingout of a defined area. Natural Increase:
The difference between the numbers of births and deaths in a definedpopulation.
In the last chapter, we learned that net migration is thedifference between the numbers moving in and out ofa defined area. Similarly, natural increase is thedifference between the numbers of births and deathsin a defined population. Once we make these twocalculations, calculating population growth is easy.
Population Growth Rate ^ Population growth is the percentage change inthe size of the population in a year. ^ It is calculated by dividing the number ofpeople added in a year (Natural Increase + NetIn-migration), per starting population, times100. Population =
natural increase + net in-migration
x 100
growth rate
starting population
Population growth rate is the percentage change in thesize of the population in a year. It is calculated bydividing the number of people added to a population ina year (Natural Increase + Net In-Migration) by thepopulation size at the start of the year. If births equaldeaths and there is zero net migration, the growth ratewill be zero. If natural increase is balanced out by netmigration, the growth rate will also be zero. In mostdeveloping countries, there are still more births thandeaths, and there is little net migration, so the growthrate is greater than zero, and the population continuesto increase from year to year.
Population Growth Rate ^ The population growth rate is difficult tocalculate using the previous formula. ^ More conventionally it is calculated using theformula below: Population=
Population(time2)-Population(time 1)
x 100
growth rate
starting population (+1)
The population growth rate is difficult to calculate usingthe previous formula. More conventionally, it iscalculated using this formula.
Population Momentum ^ Momentum is the tendency for a population tocontinue to grow even after replacement-levelfertility has been achieved. ^ Population momentum occurs because the agestructure has large numbers of women in thechildbearing years, so there are many morebirths than deaths for a long time to come. ^ Therefore, even when replacement-levelfertility is achieved, it can take the populationgrowth rate a long time to reach zero!
Population momentum is the tendency for a populationto continue to grow even after replacement-levelfertility has been achieved. This occurs because theage structure has large numbers of women in thechildbearing years, so there are many more births thandeaths for a long time to come. Even though eachwoman has only two births, there are so many womenthat there are many births, and the population keepsgrowing. So it is important to remember that evenwhen replacement-level fertility is achieved, it can stilltake the population growth rate a long time to reachzero!
Population Momentum Population momentum is mainly a function of apopulation’s age structure. In other words, the larger the number of childrenentering their reproductive years, the faster thepopulation will grow.
Population momentum is mainly a function of apopulation’s age structure. In other words, the largerthe number of children entering their reproductiveyears, the faster the population will grow. For example,in this pyramid of Country A, the momentum ismaintained by the children at the base of the pyramidwho will be entering their childbearing years.
