G E 2 4 7I N T R O D U C T I O N

T OP O P U L A T I O N

S T U D I E S

Topic 2 : Population Data, Structure and Theories–Nature and sources of population data–Age and sex population composition;

dependency ratios.–Determinants of age and sex

composition; sex ratios–Measures of population: Fertility and

Mortality–Basic population theories–Population projections

Nature And Sources of Population Data • One of the major role of demographers is to find

information on the number of people in a given time and place.

• Estimating population size is not an easy task.• It is of one of the involving work, requiring a lot of

investment in terms of resources.• In order to obtain population data, demographers

rely on a wide array of instruments and institutions to carry out their work.

• Population data can be generally divided into two main categories which are stock and flow data.

1. Stock Data Stock data is the one that shows the state of

population in an area at a given point in time.2. Flow Data This is the one that shows how the population

changes with time.THE NEED FOR POPULATION DATA.

• In any given country, population data is required in order to provide information on the following.

The number of people and their distribution within that particular country.

Population characteristics, for different populations might have different characteristics.

The current trends, levels, and patterns of fertility and mortality.

Migration patterns within and outside the country’s borders.

SOURCES OF POPULATION DATA.• There are several major and minor methods

that have been adopted and are therefore used by demographers to obtain population data.

• The major ways includes 1. Census 2. Sample surveys 3.Vital Registration System 1. Census • At a simple level, a census is a straightforward count

of the number of people in a country, region or a city.

• UN (1967), defines a census as the total process of collecting, compiling and publishing demographic, economic and social data pertaining at a specified time or times to all persons in a country.

• This therefore, implies that, though the exercise may sound simple in its definition, censuses however are not usually so simple.

• This is because, most census are also directed at gathering other information about people such as people’s economic and social information, such as, marital status and income, education levels, previous residences, etc.

• The UN has given the essential features that a population census need to have, which are as follows.

1. It must be carried by the government.2. Individual enumeration, i. e. all people must be listed

with their specified characteristics.3. Universality within a territory, i. e, it should cover

the country’s entire territory.4. Simultaneity, i. e. popn enumeration should refer to

one, well defined point in time. e.g number of people in a given night.

5. Defined territory, i.e A census should relate to a defined territory so as to allow comparison with previous census.

6. Defined periodicity. e. it should be taken at regular intervals e.g every ten years or so for easy comparison and population projections

DE JURE AND DE FACTO CENSUS• Enumeration of people in a census may be done

using a de facto method or de jure method.1. DE FACTO METHOD• In this method, a person is counted wherever that

person is found at the time of census.• This method is recommended by the UN.2. DE JURE METHOD• In this method, a person is enumerated at his or

her usual place of residence, irrespective of where they were at the time of census.

• NB: Each country have to decide on the method to use.

• In most African countries, census was and still is the main source of obtaining population data.

• In most of these countries, census has passed into different stages from colonial periods, where census was carried out mainly for colonial administrative purposes and for the aim of obtaining human resource for various areas and fields of population

• Therefore, the census were at the favour of colonialists and not to the native people.

• In 1969 the African census programme was formed, the purpose was for marking, identify resources and identifying demographic strengths of the nations.

• Even after the abolition of slave trade, most African countries realized that census is important for statistical needs.

• Census data was needed in order to make resource allocation, provision of public services and other uses.

• Due to the needs, most African countries opted on taking census for their own uses and the exercise gained popularity.

• Census exercise therefore was adopted by many African countries as one of the important exercises for any given nation.

• But with few resources and expertise, most census exercises were confronted by some problems.

• These were like, problems of census mapping ,census tests enumeration procedures, quality control and definition of various terms.

• The quality of census data in Africa has improved with time.

• Despite the fact that most of African countries are now able to carry out periodic census, still a lot remain to be done.

• Most of African countries are still not collecting accurate data on various issues that characterise population. These are like fertility and mortality data, education, employment, migration, etc.

• Therefore periodic census taking is very vital because population is very vital in environmental issues for environment sustainability. Generally census is vital because of sustainable development and environmental management.

• Advantages / Uses of Census Data.Census are provide useful information to the

government on various issues, which are.To know the actual number of people in a given

tome and spaceThis will help in resource allocation and provision

of other public services like schools, hospitals, etcCensus helps to know social characteristics in

terms of births, mortality, marriages, divorces, migrations etc.

To plan for administration patterns and other political aspects within a country, and collection of taxes and other issues.

• Information from census will help to know the actual human resource in terms of economic production s and dependency.

• Census data is useful for estimating future population trends of a given society.

• To know the current trends on the use of natural resources and the environment at large, and this helps for planning for environmental conservation and conservation measures to be taken.

Generally, information obtained from census is vital for any nation to have sustainable economic, social and political development.

Limitation of census • Census enumeration are an extremely expensive and

labour intensive undertaking for any governmental jurisdiction as a result they occur rather infrequently usually no more than every 5 or 10 years.

• The incompatibility of enumeration dates makes comparison among countries difficulty. Language barrier might be an obstacle to some of enumerators

• There is no a census which is entirely comprehensive as all census tend to under represent nonmainstream kinds of household as well as homeless individuals

2. Sample Surveys • This is the second and most important source of

population information. • This is a method in which information is collected

only from a portion of a population.• It is a kind of mini census which is small, well

designed and well administered. It is less expensive and more accurate than the census. Its reliability depend on sampling methods and the recommended one is that which uses probability sampling methods.

• Sample survey are done periodically , especially during inter census period in order to get information on population characteristics like population migration, fertility, mortality etc, and they are used to check the accuracy of census data.

Types of Sample Survey.There are two types of sample surveys1.Single round survey 2.Multi round survey1.Single Round Survey• These are always done once in a periodic time.

AdvantagesSimple to conductLess expensiveFlexible for easily administrationDisadvantagesUnder or over reporting of informationIt has non sampling error ( it is somehow biased)2.Multi Round Survey• This is a kind of sample survey in which, a sample is

choose and used as follow up surveys over a period of time.

• Here repeated visits are made to the selected respondents to ascertain what events have occurred during the period between the visits.

Advantages.• It is useful in providing information that can be

used for checking and correcting previous collected data.

Disadvantage• Expensive and difficult to administer

Uses Of Sample Survey Information • To collect vital statistics where official registration

system is inadequate or hasn’t been existed• To collect supplementary data where it is not

possible to collect the same information during census.

• To test the accuracy of the census information• To estimate the total population and its distribution• It provide information in-between the census

Limitations Of Sample Survey• Possibility of making error in sampling and hence

become less reliable• There is a possibility of being biased rising from the

design of the survey or failure to design properly that may lead into some sub group be omitted from the sample

• Sample survey is most used in developing countries because they can not afford census

3.VITAL REGISTRATION SYSTEM/ VITAL RECORDS • Vital events are those that affect population its size

and composition. Vital registration system is among the major sources of population information.

• The UN defines vital registration system as a system that include the legal registration, statistical collection, recording, compilation and analysis, presentation and reporting of the statistics patterning to vital events which include live births, deaths, foetal death, marriage divorce, abortions, separation and migration.

• These data are collected , and records of them are kept by all levels, from local to country level.

• To these authorities, vital records is a compulsory process of recording population characteristics.

• The recording is mainly done by the government registrar but also it can be done by other governmental agencies like schools, hospitals, police departments , prisons, Immigration departments,

• For vital registration to be useful and precise it must cover the whole country, and if it doesn’t , it can bring incorrect information.

Objectives• The main objective of vital registration is just to

meet the national data requirements and to collect identical information which is comparable to that of sample survey.

• There are two topics/ categories covered• The topics that describe the characteristics of the

event itself• The other deals with topics that describe

characteristics of persons involved direct in the events specifically parents.

• The birth registration include characteristics of the event or child such as date of occurrence registration name, sex, type of birth (normal or operation ) and place of occurrence, etc.

• It also include characteristics of the parents such as date of birth, name , date of marriage, occupation, usual place of residence, and names as well as ages of previous children.

• The death registration records information related to the name, age sex, and marital status, occupation place of birth date and cause of death.

• The marriage registration information include date of birth, occupation, the regions place of birth, usual places of residence, previous marital status of both partners together with date and place of marriage.

• The divorce registration information include date of divorce, date of birth of partners, occupation and usual place of residence.

• Data for international migration include information on age, sex, marital status, occupation, nationality of the migrant the purpose of visit and expected time to stay

Uses Of Vital Registration• It helps to measure rate of population change

especially in mortality and fertility it provide floor of information.

• It provide information which is important for social and economic planning i.e. education, health, voting etc

• It provide valuable record about individuals in a society

• It provide legal documents that an individual may need i.e. birth certificates, death certificates and these help an individual to inherit or pursue his or her own carrier , to claim allowances and insurances, etc.

• It provides information that can be used on check on census enumeration especially on places where there is under enumeration.

• It help to up date information on every change that is occurring on population for planning purposes

Limitations of Vital Registration System• Over or underreporting of information• It is very efficiency in urban that in rural areas• Miss statement of the age especially the sick one or

one who has died

Therefore, vital records generally are useful in provision of current information about the characteristics of a given population in a given period / time.

• Advs of vital registration over Census ans sample surveys

POPULATION COMPOSITION • Population composition simply refers to the sub

groups that constitutes a given population.• Population composition summaries the population of

a country or world by age and sex. • Age and sex are two principle characteristics of a

population. • Sex a condition of being a male or a female, and age,

related to number of years individual males and females have in a given population.

• Understanding population composition enables geographers to gather important information about population dynamics (population changes with time).

• These two characteristics of population are important because they bring about variations in the population.

Foristance, knowing the number of women of childbearing age in a population, along with other information about opportunities and status, can provide valuable information about the future growth potential of a given population.

• Therefore, understanding population composition is vital as it can tell much on future demography of a given region as well as the present demographic status.

• Many factors determine the age – sex composition, but the most important factor is fertility.

1. Fertility.• This is the most important factor in determining

population composition.• A population with high birth rate will have a large

proportion of young people while a population with how birth rate will have a small number of young people.

2. Mortality• It determine the composition but in the minimal

way. Except when there is especial case like if there is war, which wipes out certain age groups and certain sex.

3. Migration• This also determines population composition I a very

minimal way. • This is because migration is also age and sex selective,

where individuals in certain age groups and sex are liable to migration than others.eg youths groups.

AGE – SEX PYRAMIDS.• A population age - sex pyramid is a graphical

representation of all individuals in a population by using a bar graph.

• It is a representation of the population based on its composition according to the age and sex

• An age sex pyramid is therefore a bar graph which is displayed horizontally, where males are portrayed on the left hand side of the vertical axis and females on the right hand side of the vertical axis.

• Age categories are ordered sequentially from the youngest at the bottom of the pyramid to the oldest at the top.

• A population pyramid is a two bar graphs, back to back with vertical centre lines.

• The age is shown along the vertical axis and the population is shown as percentage on the horizontal axes on both sides.

Diagram

Age sex Population Pyramid of Ethiopia, 2000.

• NB. The shape of a population pyramid can differ slightly with places and time from the basic shape described above. They may differ in arrangement of the items, while the items are the same.

• eg

• The use of population age-sex pyramid is the best way to present population data.

• A population pyramid shows the demographic history, allow demographers to identify changes in the age and sex composition of populations, and provides an opportunity to demographers to access the potential impacts of a growing and declining populations.

• The shape of the pyramid can tell us a great deal about the present and future of the population

• There are several types of population structures. These are discussed below.

Types of population structureThe are five distinguished population structurea/Type 1• Is a triangular shape pyramid, which is

characteristics of those countries that have high birth rate and high death rate.

• Neither of the two (birth and death) shows decline, it suggest more individuals who are youth, and it has small old age group, but show a rapid population growth.

• Pyramids of this type were more common • During 17th and 18th centuries, and they do not

exist in today's world.

2. Type 2• It is concave shaped population pyramid, showing a

population where death rate is declining while birth rate is remaining high it constitute a large population being young. eg less than 15 years.

• It shows a population where death rate is declining whereas birth rate is still high.

• It has a narrow top and a wide base. • It has a high potential for future population growth

due to reduced child mortality, and therefore more young people will be able to enter child bearing age.

• It is a characteristic of many developing countries today, eg Tanzania, Kenya, etc

c/ Type 3• This is a convex shaped pyramid, which is a narrow

based pyramid with convex sides.• It indicates low population growth and population

becoming old.• It is associated with countries having low birth rate

and low death rate, eg Sweden, British.

d/ Type 4• This is a bell shaped pyramid, which shows a

population where death rates are declining• It is found in countries like Canada, and USA which

have experienced baby boom (abrupt increase of children).

• The marked narrowing in the reproductive age period indicates a decline in the birth rate at an earlier time.

• However, there is a renewed increase in birth rate after a considerable period of low birth and low death rates.

e/ Type 5• It is a pyramid which shows a population where

death rates are declining.• It also shows a population that was once growing

very rapidly and then experienced a dramatic decline and continued decline in birth rates. eg Japan.

MEASURES OF AGE SEX COMPOSITION.1. Dependency Ratios• Dependency ratio refers to a measure of the

economic impact of young and old people on the more economically productive members of the population.

• Often is divided into three distinct groups, which are :

Young Cohort / Young Population, comprising of those aged less than 15 years.

• These are considered to be too young to be fully active in the labour force and they are therefore economically inactive.

Middle Cohort / Productive population comprising of those between 15 – 64 years.

• These are considered to be economically active, as they are actively involved in labour force.

Old Age Cohort / Aged population , comprising those aged 65 and above.

• These are beyond their economically active and productive years.

• By dividing the population into these three groups,it is possible to obtain a measure of the dependence of the young and old upon the economically active, and the impact of the dependent population upon the undependent.

• Dependency ratio is can be easily obtained through the following mathematical relationship.

Popn 0 -14 + popn 65+Dependency Ratio = ………………………………… x 100 Popn 15 – 64

• Dependency ratio measures the number of dependants that each 100 people in the middle cohort must support.

• Depending on the population structure and composition, each country has its own dependency ratio.

• Some countries have a high dependency ratio, because of having many individuals in the young and / or old cohort.

• Typical examples are found in some developing countries, where birthrates are high and death rate have decreased.

• Other countries have low dependency ratio because of the few individuals in the young and old cohort.

• Developed countries shows a low dependency ratios.

• A high dependency ratio means less servings and therefore little or no money will be available for investments, while with a low dependency ratio, there will be serving and investment will be possible.

2. Sex Ratios• This refers to the ratio of males to females in a given

population.• It is the number of males per 100 females in a

population.• Sex ratio can be easily obtained for a given

population by using the following formula.

Number of malesSex ratio = ………………………………………… x 100 Number of females

• The sex ratios are important to demographers because they can affect the population dynamics in in a given population.

• Sex ratios indicates the followingThe sex ratios of 100 indicates that there are equal

numbers of males and females in a population.• In this kind of population, there is a great probability of

population increase, as these people reach reproductive age, it will be easy to find mates.

A ratio of over hundred indicates that there are more males than females, a condition which predicts a slow population growth.

A ratio of below hundred indicates that there are more females than males.

• The naturally occuring sex ratio in human population at birth features a slight predominance of males than females, i. e. for every 100 female infant born, 105 to 106 males are born.

• Sex ratio is normally affected by mortality, migration and fertility.

• At every age, males have a higher death rate than females.

• Foristance, long distance migration is preferred most by males than females, who prefer short distance migration.

• Other catastrophes like wars tend to wipe more males than females.

• Selective abortions on the other hand have got significant impacts in the population. This is because some couples will selectively abort a given fetus of a given sex due some cultural dictations.

• There are several measures of population, which may include

Count – The absolute number of a population or demographic event (eg a birth) for a specific time and place.

Rate – The frequency of a demographic event in a population for a given time period.

Ration – The relation of one population sub group to the total population or to another sub group

MEASURES OF POPULATION• In order to arrive at different understanding of

population growth and change, experts look at two significant factors which are fertility /natality and mortality.

• This is because the two are important measure of population change in a given society.

• Birth and death rates, along with other factors like migration help to determine whether a human population grows, shrinks or remain fairly stable.

The Concept of Fertility.• Births are the primary means by which numbers are

being added to the existing population, and therefore it is an important demographic variable in overall population growth.

• Fertility/ Natality refers to actual bearing of children. It refers to the number of individuals added to the population through reproduction.

• Fertility rates refers to the relative frequency with which births occur within a given population.

• Therefore, fertility is the actual number of live births born by a female.

• Live births reset to any child born alive. Fertility occurs over time and therefore it can be measured.

• Fertility differs from fecundity in a sense that fecundity is the biological capacity to bear children. i. e. the potential to give birth.

• Fertility can be looked at with regard to the following

• Marital fertility which refers to fertility that occur within marriage and it counts for greatest percentage of all fertility.

• Teenage fertility, which refers to fertility that occurs to teenagers.

• Non - Marital Fertility which is any fertility that occurs outside marriage boundary.

• Overall fertility refers to total fertility, whether within or outside marriage boundaries.

• Fertility is an important factor in demography simply because it can happen more than once to the same individual, while death happens only once.

• However, fertility is less predictable than mortality mainly because it is linked to a wide range of cultural and social variables and to the level of modernization.

MEASUREMENTS OF FERTILITY.• Fertility can be measured.• For convenience, fertility is measured by accessing the

rates that shows the frequency with which it occurs.• There are several measures of fertility, and these are

as follows.1. Crude Birth Rate,(CBR).• This is the ratio of the number of live births in a single

year for every thousand people in a population.• It is called crude because it measures the birthrate in

term of the total population and not with respect to a particular age specific group/cohort.

• To calculate the CRB, take the total number of births in a year for a region and divide by the mid – year population then multiply by 1000.

CBR = Number of live Births ...................................... X 1000 Total population

• The CRB of any given society/country is affected by many factors, which includes the level of economic development, women’s educational achievements, religion, social customs, diet, health, political and civic unrest, etc.

Foristance, if the total population of country X in a given year X is 40 000,000 people and the number of live births in the same year is 4000 births, then

4000 CRB = ..................... X 1000 = 10 40,000,000

The CRB will be equal to 40. (i. e 40 children per 1000 people in a population.

• The major weakness of this measure is that it assumes the involvement of the whole population.

• It includes even children who do not have the biological capacity to bear children ,males and elderly people who are in their post reproductive ages.

• Therefore it is not an inadequate measure of actual fertility because its denominator includes many persons who do not contribute to births.

• If a population consists of a higher proportion of children, males and older people, then the CRB will be lower than its actually supposed to be.

2. General Fertility Rate, (GFR)• This refers to the number of live births per 1,000

women aged 15 – 49 which is the typical cohort of child bearing years in a particular year.

• To calculate the GRF, the total number of births in a year for region is divided by the total number of women age 15 – 49 in the same year (i.e those “at risk of giving birth), then multiply by 1000.

Number of live birthsGFR = ....................................................... X 1000 Number of women aged 15- 49

For example, if a given population has about 215 live births per year and the total number of women of child bearing ages are about 22500, the GRF will be as follows.

Number of live births per yr• GFR = ………………………………………………... x 1000 Number of women of 15-49 yrs = 215/22500 x 1000 = 9.5/1000

i. e, there are 9. 5 live births for every 1000 women in child bearing age.

3. Age – Specific Birth Rate, (ASBR)• This refers to the number of live births to women in

a give age class, like 15- 19, 20 – 24, 40 – 44, etc.• It is obtained by taking the total number of birth to

women in a particular age group and divided by the total number of women in that age group.

Number of live births by x age groupASBR = .......................................................... X 1000 Number of Women of x age group

• Foristance, live birth to women 15 – 19 were 200, while the total number of women aged 15 – 19 were 16,665.Then

ASBR = (200/665) x 1000 = 12/1000

• This means that for every 1000 women in that age group, there were 12 live births.

• ASBRs are useful because child bearing varies considerably with age, i. e it is not evenly distributed over the whole period from 15 to 49 years.

• Therefore, the ASBR brings out the differences in fertility that is due to age.

• Child bearing begins gradually and reaches its peak in late twenties and early thirties, from there, it declines slowly among some populations and more rapid among others.

• ASBR is more referred measure of fertility because the dominants include the women only those on their child bearing ages, who are at the risk of having live births.

3. The Total Fertility Rate (TFR).• This is a measure of the average of number of

children a woman will have throughout the years of her childbearing ages, which are 15 to 49.

• It is a more predictive measure of fertility than CRB, that tries to portray what birth rates will be among a particular cohort of women over time/ during her life time.

• Replacement fertility , on the other hand is the TFR that keeps the population stable.

• For humans, the RF is equal to the TFR of 2.1.• A TFR higher than 2 implies the replacement and a

stable population, but below 2, and in absence of migration, the population will shrink.

• This is an artificial measure, as it does not apply to any individual woman. It is a way to asses the child bearing habits of a typical woman using the dominant child bearing habits of all women in a society at a given time.

• Here the age specific birth rate is not multiplied by 100 or 1000 but by five. Multiplying each of the rate by five provides the number of children she would have for each five years period. Summing up the rates for all age categories results in the number of children she would have by age 49 , which is the total fertility rate.

SELF READ.READ & WRITE ON

FACTORS AFFECTING FERTILITY.

The Concept of Death/Mortality.• Mortality refers to deaths occuring in a population.• Death is defined a the permanent disappearance of

all evidences of life at any time after birth has taken place.

• Death is one of the life certainties, as every individual in a population will eventually die at one given time.

• Though death is certain, the length of time a person lives depends on many factors.

• Some people die sooner than others, and generally, death comes earlier to males than females.

Causes of Mortality.• In any given population, mortality varies from time to

time and from place to place.• The UN has grouped its causes into five major groups.Group 1 – Includes all deaths which results from infectious, parasitic and respiratory diseases.Group 2 - Deaths occuring due to cancer.Group 3 - Deaths occuring from diseases of circulatory system e.g. BP, Hypertension, e.t.cGroup 4 – Deaths occuring from violence including accidentsGroup 5 – Deaths occuring due to other causes, e.g birth injuries, diabetes, etc.

• The prevalence of each cause of death is strongly related to overall level of mortality.

• As the average life expectancy of a population increases, the proportion of deaths due to group 1 diseases declines.

• The proportion of deaths due to group 2 and 3 increases slowly until life expectancy is sixty and then increases sharply.

• The proportion of deaths due to group 5 diseases increases slightly until life expectancy reaches 70 years and then declines sharply.

• Deaths attributed to violence or accidents do not seem to be related to overall mortality.

• The pattern shows that, as a society modernizes, the infectious and parasitic diseases are brought under control and deaths declines.

• This is due to modern medical services and the improved standards of living of people.

• At the same time, the proportional of deaths due to degenerative diseases increases significantly, as observed from most developed countries.

• Causes of deaths can also be explained as follows.1. Congenital malformation and genetic errors of

reproduction and diseases.2. Famine or malnutrition and accidents.3. Wear ant tear of vital organs due to old age.

1. Group one diseases are responsible of the early mortality during infancy and childhood

2. Deaths in the second group occurs during the later part of infancy and early part of childhood. Most of them are attributed to infectious or parasitic diseases.

Scarcity of food, increases mortality rated to infants, children, pregnant mothers and old people.

3. The third group affects more later stages of lifespan. Though they are automatic, but human behaviors can accelerate them e.g. food, leisure, alcohol and drug use, smoking, environmental pollution e.t.c

NB. Causes are also explained by the epidemiological transition model which will be studied later.

Mortality Differentials.• The timing of death varies among categories of

people ,where the risk of dying differs with age, sex , race and economic status.

1. Age• Mortality is closely related to age, where death

rates are more to infants under age 1, and is low for children, adolescents and young adults and the increases in old ages.

2. Sex• Death risk appears to be more to males than

females at all ages and in all populations, therefore, males have a higher death rates than females.

• It is argued that, this is brought by the biological differences between males and females where females are biologically superior to males

• The other reason is that males are deployed more in hazardous occupations than females.

• Females lives about 4 years longer than males and in developed countries, this difference is as high a 7 years.

3. Economic Status• The higher the social economic status, the lower the level

of mortality.• Coupled with occupation and levels of education, the

social economic status of the population is an important aspect in mortality because, it reflects the differences in income/earnings, which in turn affects a number of life related facilities.

• Differences in income and earnings leads to differences in access for health facilities, diets and general nutrition, clean and safe environments, amount of stress and strain that exists within social and economic strata e.t.c.

• Occupational differences also brings about differences in mortality rates in a given population, lifestyle differences associated with membership to various social economic strata are also factors for differences in mortality rates.

• Education on the other hand is an important aspect of differences in mortality rates in a population.

Measures Of Mortality.• Several methods have been devised by

demographers in order to have mortality statistics.• These includes the following. 1. Crude Death Rate (CDR)• This refers to the number of deaths in a year per

thousand people in the population • To calculate, take the total number of deaths in a

year for a region and divide by the total population

Number of deaths in a yearCDR = .................................................... X 1000 Total population in a year

• It is a crude measure because the total population is taken as the denominator, whereas the probability of dying in a particular period is not equally foe everyone in the population.

• It is therefore affected by a number of factors including the age sex composition of the population and other population characteristics.

• Age structure affects CDR, where older population may have high CDRs, but these reflect the age structure not the health conditions or other factors.

• However, CDR is useful as it gives a general indication of the levels of mortality in a given population.

2. Age Specific Death/ Mortality Rate (ASDR)• This refers to the number of deaths that occur

within a specific age in a given year, e.g. 0 – 4, 5 – 9, 30 – 34, e.t.c.

• It is amore refined measure of death, because death is more likely to dominate more in some ages.

No. of deaths in a specific ageASDR = …………………………………………………… X 1000 Total Population in the same age

• This rate is computed separately for each sex.• ASDR is more accurate measure of death because it

takes consideration of specific age groups.• Generally, death rates are lowest for adolescents

and young adults and the rates are highest in infancy and in early childhood as well as in old ages.

3. Infant Mortality Rate (IMR)• This refers to the number of death of infants under

age I per 1,000 live births in a given year.

No of deaths of infants under age 1 IMR = ...................................................... x 1000 Total live births

4. Cause Specific Death Rate, (CSDR).• This measures the importance of certain cause of

deaths in a given year.• It gives the percentage of deaths due to a certain

cause as opposed to total deaths in a year.

Deaths due to a certain cause CSDR = …………………………………………… x 1000 Total Deaths in a year• This measure helps to show which cause is

important in causing deaths and therefore it gives an opportunity of eradicating it.

Life Expectancy.• Life expectancy refers to the average number of

years one could expect to live if the current age – specific death rates remained the same for the rest of his or her life.

• Life expectancy varies by sex, age and other factors. It is usually cited separately for males and females.

• Life expectantly at birth – Average number of years a baby born this year can expect to live if current specific death rates remain the same.

• Life expectancy is a good indicator of health condition and a general estimate of population of individuals in a given population.

FACTORS THAT CAN DETERMINE IF WORLD POPULATION GROWTH RATE IS RAPID INCREASING.

Population growth rate between years e.g. Between 1880s to 1890 1990 – 2005• This can show if it is increasing or decreasingPopulation size from various census is also an

important factor for determining whether a population is changing or not. Also other forms of census data will give the same information.

Example, from various census we are able to conclude that the population of Tanzania is growing, as it is shown by the figure below.

Doubling time is another indicator which refers to a period over which a population will double itself.

• The standard time to calculate the DT is to assume that the population grows exponentially(i. e. Has a constant growth rate).

• DT is estimated by dividing 70 by the annual growth rate which is stated as percentage.

• However it is important to note that, DT is very sensitive to growth rate, and it changes quickly as growth rates changes

• For example if the population is growing at 1% it will double after 70 years, and if it grows at 2 %, the DT will be 35 years.

• If the growth rate increases the doubling time decrease for example in Tanzania, growth rate is 3%

Therefore, the DT = 70/3 = 23 Years.

BASIC POPULATION THEORIES.• A term population theory refers to the body of

generalizations and population principles found various disciplines within social science.

• These generalizations consists of coherent set of hypothetical and conceptual principles forming the general frame of reference for population studies.

• Population theories attempt to explain major factors determining population growth and /or demographic characteristics of a given population.

• A population theory should satisfy certain conditions which includes the following.

a) It should consist of a set of hypothesis or principles b) It should explain historically the observes changes

in fertility, mortality and migration patternsc) It should be able to provide the basis for predicting

with some accuracy future levels and trends of population growth as determined b stages of socio - economic development.

d) It should be able to help us understand the relationships between changes in mortality, fertility and migration.

History of Population Theories.• Historical evidence suggests that man has been

concerned with population problems since ancient times.

• Thus the history of population theory may be divided into three phases which are

a) Pre Malthusian Period. b) Malthusian Period. c) Post Malthusian Period.

Post Malthusian Period.• These are theories that were developed afterMalthus and Karl Max. They include the following.Biological Theories of population Sociological Theories of PopulationThe Demographic Transition Theory

SELF READREAD ON THE FOLLOWING

THE DEMOGRAPHIC TRANSITION MODEL• The demographic transition model describes a

sequence of change over a period of time in relation to birth and death rates and overall population change, ( i.e. natural increase).

• The theories describe and predict the future population of any area (place).

• It is based on the population changes in several industrialised countries in Western Europe and North America.

• The model assumed that, all countries pass through similar demographic transition or population cycles.

• The model tell us that, the population of any region changes from high births and high deaths to low births and low deaths as a society progresses from rural agricultural and illiterate to urban industrial and literate society.

• The theory have its roots from 19th century, where through various studies it was noted that fertility levels were falling in many western countries.

• These demographics studies gave birth to what is called demographic transition theory.

• The main contributors to the theory are Landry W. Thompson, C. P. Blaker, Kingsley Davis and F. Noteisteint .Their contribution to the models is as follows.

• Landry published a paper in 1909,where he identified three main stages of population growth which are

a) The primitive stage, in which fertility is not restricted at any cost.

b) Intermediate stage in which fertility is in the process of being restricted through postponement of marriages.

c) Modern stage in which mortality rates are declining due to conscious efforts being made to limit family size.

• However, he did not develop his description into a theory with universal application.

• W. Thompson (1929) and C. P. Blacker (1947) made attempts to generalize the demographic experience of Europe into a theoretical framework which they believed could work for other areas as well.

• W. Thompson divided the nations of the world into three categories according to their levels of mortality and fertility.

a) The first category comprised all countries with high birth and death rates.

b) The second category includes all countries characterized by declining rates in fertility and mortality.

c) The third category includes countries experiencing decrease in population growth rates due to rapid decline in fertility and mortality rates.

• On the other hand, Blacker identified five stages of demographic transition which are as follows.

a) The first stationary stage characterized by high birth rates and death rates.

b) The second, early expanding stage with high birthrates but with decline in mortality rates.

c) The third, late expanding stage with falling birthrates but more decreasing death rates .

d) The fourth, low stationary stage with low birth rates are balanced with low mortality rates.

e) The fifth, declining stage where mortality rates exceeds fertility rates.

• Contributions of these scholars is highly appreciated, as their work formed the basis of the modern demographic transition theory we know today, which was developed in its mature form by another scholar, who is Frank Notesteint in 1945, and expanded it in 1953.

• Due to his work, he is regarded as a father of demographic transition theory.

• Through his observations on the population of Europe in 19th century, Notestein relates mortality and fertility levels to fundamental economic and social forces of modernization experienced in Europe in that time.

• Many demographers believe that fertility and mortality rates are directly linked to the level of economic development of a country, region or place.

• Therefore, demographic transition theory was developed in respect to the history of demographic change in core countries, where they contended that many economic, political, social, and technological transformations associated with industrialization and urbanization lead to a demographic transition.

• The model also gives reasons for transition/change at each transition stage and more specifically it gives example of countries that appear to fit each transition stage.

• According to the model, there are four stages through which a population of a given country must pass through, and they are named as stage 1, stage 2, stage 3 and stage 4, where each stage has got its own characteristics.

• This means that, population changes occur in stages which are collectively known as the demographic cycles.

• These are explained as follows.

Stage 1.• This is a first stage characterised with primitive

demographic growth, where birth rates and death rates are high and fluctuate giving a small population growth.

• Many children die in infant and hence parents tend to produce more hoping that few will survive.

• In this stage, children are seen as symbols for economic and cultural wealth.

• Economically, children are needed to work on land as most people are engaging in agriculture so large families are economic assets.

• Socially, children’s are regarded as a sign of virility (prestige).

• There are believes that discourage the use of family planning methods and a result, many are added to the existing population.

• In the past years, most countries belonged to this stage.

• Reasons to why there is high death rate includes lack of both birth and death controls.

• Conditions like infectious disease like cholera, famine ,Poor diet and hygiene, lack of health facilities, low education levels etc are typical characteristics of the stage.

Stage 2• This is a second stage where birth rates remain high

but death rate fall rapidly about 20 per thousand people and hence giving rapid population growth.

Why there is decline on death rate? Improved medical services ie availability of

hospitals, vaccination, scientific research Improved water supply and sanitation.Improvement in food production both in term of

quality and quantity.Improvement in transport and communication

network.

• All these results into the dramatic decrease in death rates and this ensures the survival of most of the individuals who are added to the population through births.

• Therefore, this decrease in death rates leads to an increased population and ensures maximum population growth.

Stage 3. • This is a third stage where birth rate fall rapidly while

death rate decline slightly giving a slowly decreasing population. The fall in birth rate is attributed by

• The decreased death rates are due to many reasons.

These are like availability and acceptance of the modern family planning methods, industrialization and mechanization higher levels of education, changes in the social patterns including women empowerment, human rights, e.t.c.

• Here, there is great desire for material possession and therefore children’s are less desired.

Stage 4. • Both death rate and birth rate remain low

fluctuating slightly to give a steady population.• The population here remains fairly steady because

deaths and deaths have amore or less similar pattern.

Possibly Stage Five?• Though the model has not stated, in reality there is a

new tendency where the number of deaths outweighs the number of individuals replaced through births, giving an overall negative population growth.

• Scandinavian countries, specifically Sweden is currently entering into negative growth rate, meaning that there are fever births than deaths so that the country’s population is declining drastically.

Limitations of The Model.The model is Eurocentric in nature because it

assumes that all countries will pass through the same stages, as it was in Europe.

Birth rates for instance in German and Sweden declined before even the model was put. This suggests the new stage to be added in that model.

There are cultural and political values that influence changes. But the model is based on social economic factors only. Foristance, it does not take migration into account as a component of population growth/decline.

The time scale is not well organized i.e. What is the estimated / specific time from are stage to the next.

It does not take into account if the pandemic diseases like AIDS, which may wipe signifanct numbers of people in a given population.

Applicability of the Model • Together with the weaknesses shown by the model,

this model is highly applicable because of the following.

The model show the population of a country changes over time.

It compares rates of growth between countries.The model has shown the factors for change in a

population i.e. diseases, industrialization, e.t.c.The model has tried to give some examples of

countries that seem to appear from one stage to the other.

2. EPIDEMOLOGICAL TRANSITION MODEL.• This model explains the causes of death.• Death is the disappearance of all evidence of life at

any time after birth has taken place.• The model explains about studying death rate in a

population. The effect of parasitic and contagious diseases have been drastically reduced in many parts of the world .

• As a result the large proportion of the population lives long enough to diseases to be afflicted by and in due cause die from degenerative diseases.

• It has been recognized that societies pass through various patterns of mobility (illness and diseases) and mortality as the causes of death.

• Mortality (causes of death) during the development process, it can be even if not at all stages and sequences are identical in every case.

• Health improves mortality and mobility fall down.• The model states that societies during their

development they have to pass in epidemological model.

• The model explains the complex changes in pattern of diseases that a population passes through and the social and biological determinants. It shows dis-placed or replaced by degenerative diseases as causes of mortality.

• The transition model is associated with various degree of development from social economic point of view to medical aspect. And therefore the result is a shift in life expectancy.

• Many people were dying at young ages but currently the majority thrives to old ages.

• The theory has five proposition or stages, which are as follows.

1. Proposition One• Mortality is fundamental factor of population

dynamism or change .It occurs at every body at various time scales.

2. Proposition Two.There is a shift in mortality and disease patterns,

where the pandemic of infection diseases are progressively delivering and now being replaced by degenerative diseases or man made diseases.

• The replacement is assumed in three stages(a)Age Pestilence and Famine. It is a stage of pre

industry where people lived in problems from environment, there was no control of calamities.

(b)Age of Receding Pandemics of infectious diseases, where the main problem were infectious diseases.

(c) Age of degenerative and man made diseases, with the improved livelihoods and medical sector, people are suffering from diseases mostly resulting from their lifestyles, feeding habits and all other forms of modernization.

3. Proposition Three• During the epidemiologic transition the most

profound changes in health and disease patterns obtain among children and young women.

• The genuine improvements in survivorship that occur with the recession of pandemics are peculiarly beneficial to children of both sexes and to females in the adolescent and reproductive age periods, probably because the susceptibility of these groups to infectious and deficiency diseases is relatively high.

4. Proposition Four• The shifts in health and disease patterns that

characterize the epidemiologic transition are closely associated with the demographic and socioeconomic transitions that constitute the modernization complex.

• This proposition in some ways bridges the other propositions in that, consistent with third, it is characterized by lower fertility and longer birth intervals, and in keeping with the second and the fifth, it is posited that, improved socioeconomic status leads to better nutrition and sanitation, which in turn improve health and reduce morbidity and mortality.

5. Proposition Five• Peculiar variations in the pattern, the pace, the

determinants and the consequences of population change differentiate three basic models of the epidemiologic transition: 1. the classical or western model, 2. the accelerated model and 3. the contemporary or delayed model.

• They roughly correspond to (1) the experience of developed countries that evidence slow declines in death rates followed by lower fertility rates that accompany modernization, (2) the experiences of some countries like Japan, where the course of transition was much more rapid and the amount of

time required to reach the milestone mortality rate of 10 deaths per 1000 is much shorter, and (3) the experience of developing countries where there have been more recent declines in mortality but not in fertility rates because infant and maternal mortality rates are still high.

• Through the description, analysis and comparison of mortality patterns in many societies and at different points in time, distinctive core patterns of the epidemiologic transition emerge.

• The fundamental purpose of delineating these models is to visualize the different matrices of determinants and consequences associated with mortality (and fertility) patterns and to elucidate some of the fundamental issues confronting population policy-makers.

• The theory of Epidemiological transition usually favours some groups.

e.g. It favour young over old, Females over males, Rich people over poor people, etc.

• This is because, foristance, children are more vulnerable to diseases than old people, rich people are less affected by mortality because they have recourses to control diseases.

• Therefore, the model argues that there people (groups) that are advantaged while other people (groups) are disadvantaged in terms of diseases.

• Though many contributed to the theory, A.M. Omran is a typically credited with formulating the theory of epidemiological transition.

• In 1971, he wrote his publication on which the following were cited most, ‘Conceptually, the theory of epidemiological transition focuses on the complex change in patterns of health and diseases, and on the interactions btn these patterns and their demographic, economic and sociological determinants and consequences.’

• Therefore, the propositions are also known as the Omran’s Five Propositions.

THEORIES OF FERTILITY• Malthus Theory• Marxist theory• Neo Malthus theory• Micro economic theory1. MALTHUS THEORYThe theory was developed by Thomas Malthus.Malthus argued that there was a positive

relationship between family size and income.There was a strong and stable passion among sex

and this could be fulfilled by marriage.

The passion between the sexes is constant. Marriage is a pre – condition for child bearing. Discouragement of early marriage through

economic and other functions affect the level of fertility .

• He assumed that agricultural resources were limited and could only increase arithmetically, i. e.,1, 2, 3,4 etc.

• On the other hand, population were increasing at a geometric progression, i. e. 1, 2, 4, 16,32,64, etc.

• With this trend therefore, people will experience hunger.

• He argued that, if population increase will not be controlled through various undertakings, natural factors will take their role.

• Malthus argued that the only way the people could escape poverty was by delaying marriage.

• Despite the good explanations made by Malthus, he failed to foresee technological change that can revolutionalize agriculture.

• He argued that since the resources are scarce high fertility always lead to poverty. And therefore this theory is called MACRO ECONOMIC THEORY.

• Because it try to explain group behaviour of fertility rather than individual behaviour of fertility. And therefore it can not predict fertility at family level.

THE THEORY OF FERTILITY BY KARL MARX• He argued that fertility is not much affected by

natural laws as argued by Malthus but it is affected by social law found in a given society at a given time.

• Marxists argue that the level of fertility is a function of existing class structure and the laws /roles assigned to different classes within a society.

• He argued that in capitalistic system, a lot of cheap labour was needed and the system encourage high fertility while in socialist system fertility will be low because there is no need of creating labourers.

• According to him, fertility was a reflection of social and political condition.

• Despite a good job done, the theory focuses more on social classes as an explanatory variable.

• Also, the theory sees fertility as a part of large economic and social system

• It sees both, biological and cultural factors playing a little role in affecting fertility.

• Neither Malthus nor Karl Marx was concerned with the effect of infant mortality on fertility.

• They both looked at fertility at the macro level (at the broadest level).

MICRO – ECONOMIC THEORY OF FERTILITY • It is based on individual differences that can affect

fertility. i.e household income, education, age, etc. i. e. individual matters that affect fertility.

• There is an assumption that fertility is related to a single family, couple, etc.

• The emphasis on these theories are on environmental constraints. They argue that biological factors are not a determinant that governs the fertility rate but it bases on income.

THE NEW HOME ECONOMIC APPROACH• This falls under the micro economic theory, and it is

an expansion of the micro economic theory, where the main argument was that, fertility discussions are economic because they involve the search for an optimum number of children in the place of economic limitations.

• Here, parents have three things to deal with and these are

The number of children they have/need to have.The quality of childrenThe quality and quantity of other goods wich they

consume.

• According to this theory, the quantity and quality of children and goods they consume are interrelated.

Weaknesses of The Theory.• The theory is static because it assumes that the

couple knows very early in marriage its future, income and occupation and the number of children they will have.

• The theory doesn’t consider contraceptic failure and mortality of children or parents.

THE SOCIAL DETERMINANT SCHOOL OF THOUGHTS• The main contributors to this theory are Leibenstein

and Easterlin.• Their major argument is that, families differ

fundamentary in the value they place on children.

• They add that biological constraint on fertility is important contributor to fertility differences. They suggested that variables like religion and education should be used as control variables.

• They argue that the combination of biological and cultural factors determines the natural fertility of the population and it is that which determine the limit of the family size.

OTHER FACTORS THAT CAN AFFECT FERTILTY• There are other factors that have been associated

with fertility and they do with.The percentage of women that remain single The level of education of the family.

The occupation of the femaleReligion or religious status of the couples

Transitions In Epidemiology And Public Health.• It is instructive to consider historical transitions in

epidemiology and public health that in some ways mirror the transitions described above.

• The modern public health movement had its origins in the Sanitary Movement of the 18th and early 19th centuries, with its focus on community characteristics, economic conditions, and environmental influences.

• Improvement of living conditions was seen as a means of improving health.

• This formative period in the history of public health continues to be relevant, reflecting a rudimentary version of the current understanding of the determinants of health as multifactorial and contextual.

• By the end of the 19th century, the germ theory of diseases had matured and largely displaced the miasma theory on the basis of scientific advances in bacteriology, chemistry, and medicine as well as epidemiology.

• The recognition of infectious diseases as major contributors to morbidity and mortality, the rapid development of new knowledge and tools, and the effectiveness of public health efforts in reducing the incidence of and mortality from some infectious diseases led to increasing dominance of this theory, which dramatically shaped public health programs and practices, and contributed to steep declines in mortality from infectious diseases in the U.S. and other developed countries.

• The other side of this development was that public health came to be viewed largely through the lens of disease prevention and control.

• Consistent with the transition theory, by mid-20th century, public health attention shifted to chronic disease prevention and control, with emphasis on risk factor epidemiology and interventions directed toward individual behaviour and lifestyles.

• The earlier dominance of germ theory and microbiology may have narrowed the view of the mission of public health, putting the population focus and developing quantitative approaches in

competition with the microscope, but new developments in understanding genetic and cellular processes,(a potential new “germ theory”) have been accompanied by renewed interest in both psychosocial characteristics and broader contextual and environmental influences.

• This broader perspective on the determinants of health and disease encompasses but goes beyond traditional risk factor epidemiology.

• It includes explicit attention to the complexity of systems and the challenges of integrating multiple levels from the genetic to the personal to social, political, and economic contexts across the life span.

• This methodological transition has been particularly evident in the remarkable evolution of epidemiology as a discipline since World War II.

• These changes have resulted from developments on several interrelated fronts, both within and outside the field. Theories of health and disease have received greater attention, with broader concepts of health as encompassing more than the absence of disease.

• A renewed focus on population health, as well as development of more complex causal models reflecting new discoveries focused greater attention on more thoughtful considerations of causal inference and explanation of causal associations, not just identification of risk factors.

• The continuing threat from infectious diseases and increasing attention to health disparities have challenged traditional risk factor approaches.

• Some of the more recent discoveries in infectious disease have arisen from an approach and conceptual framework and methodology rather distinct from that of the golden age of late 19th and early 20th century microbiology.

• Awareness of the importance of a life span perspective has also been accompanied by increased attention to etiologic investigations(scientific studies of diseases causes) of degenerative diseases of aging formerly thought to be unavoidable.

• Alongside, explorations of genetic causes has been renewed interest in research on environmental factors, both as external causal agents and as potential modifiers of genetic causes, with the goal of understanding processes in order to develop more effective preventive interventions.

• While retaining intervention and prevention as goals, epidemiologists now seek to incorporate systems perspectives within an ecological model with its multi-leveled approach and life span considerations.

• In sum, the eco-epidemiologic approach is characterized by a life course perspective, recognition of multiple, interrelated levels of causation, and an emphasis on models that are more integrated rather than fragmented.

• The implications for research methods parallels those three components.

• First, the life course perspective requires us to think in terms of changes in causal pathways across the life span when considering shifts in the age distribution of a population as described by the epidemiologic transition theory.

• Second, the causal models on which we rely must allow for multiple levels of determinants acting in complex and interrelated ways, often synergistically or with feed-back loops or reciprocal lines of causality.

• Third,we have to consider that higher level or “upstream” determinants may have emergent properties that are more than the aggregate of their constituent parts, so that we must consider them along with the lower level elements in our models and analysis.

• Finally, when considering the multiple levels of the ecological model, we rely on the understanding that disease occurs in individuals, but interventions can occur at any level, including communities.

• The 2002 Institute of Medicine follow-up report emphasized the “public” aspect of public health, that is, “healthy people in healthy communities.”

• This in some ways is a recapitulation of the Sanitary Movement’s emphasis on living conditions, but goes beyond that earlier perspective. There has been a rich discussion in the public health literature on the definition and nature of healthy communities.

• The critical development is that, public health professionals increasingly recognize an organic notion of community, emphasizing that individual health is achieved or threatened by larger scale contextual factors, including social networks, environment, education, economic opportunity, and other characteristics of communities.

• This emerging perspective sees the whole range of determinants as integral to personal and community health and well-being.

• The modern ecological model of public health practice stresses the multiple dimensions that constitute our lives, relationships and environments, and, therefore, contribute to health and wellness or disease and disability.

• Further, the threat of emerging infectious diseases continues to have global significance in an era of resurgent multidrug resistant tuberculosis, pandemic such as AIDS, and widespread distribution of vector borne diseases.

• These demonstrate that the transitions cannot be neatly categorized into either historical periods or geographic locations.

Critiques of the theory• The epidemiologic transition theory appears to have

some confirmation in recent trends that were characterized by increased life expectancy and a shift in the population age distribution to older ages as well as the concomitant increase in the numbers of people living with chronic degenerative disease.

• These changes have profound impact on public health planning, health care resources and workforce development, and a range of social, political, and financial policies.

• While such changes consistent with the theory have been evident, the theory has not been without its critics and a number of issues remain controversial.

1. The theory fails to grasp the global nature and historical sequence of the mortality transition as it spread.

• Criticisms of the original theory reflect continuing development in theories of health and disease, disagreement about the role of advances in medicine relative to public health interventions, and debate about the relative importance of various contributors to the unquestioned changes in mortality and disease patterns, especially with regard to nutrition, poverty, and income inequalities.

2. Another argument is that, the emergence of cities and organized societies that triggered the first transition came with social stratification ,that is a precursor of the disparities in social position and wealth observed in later stages.

• The argument is that, wealth and poverty and their relative distribution play central roles in health and well being in each stage.

• This was argued by Pearson, that, the roles of income and education in the most recent stage are paradoxical in that, compared to those who are poor and less educated, wealthier and more educated persons tended to be earlier adopters of

lifestyles that contributed to the increased risk of those diseases that now constitute the major causes of death, but they were also earlier adopters of treatments and lifestyle changes that subsequently reduce the risk of morbidity and mortality.

• The result, according to Pearson is that poor and poorly educated populations may experience later peaks in the incidence of the diseases that define the third transition phase, but also continue to suffer from elevated rates of those chronic diseases after rates have begun to decline among those better off.

• Pearson argues that this requires simultaneously working for economic development and improved education to reduce prenatal, infectious, and nutrition-related diseases while implementing proven strategies to discourage adoption of those detrimental behaviours and exposures associated with chronic diseases in more prosperous segments of the population.

• Note that these differences are most evident in comparisons of developed and underdeveloped countries, but also hold for different segments of the population within the same society or country.

• It can be argued that, the publication of Omran’s paper came at a time of naïve optimism concerning mortality patterns, causes of morbidity and mortality, and global progress in life expectancy.

• In 1969 testimony before congress, the US Surgeon General said that it was “time to close the book on infectious disease as a major health threat. ”

• Though it might have been impossible at that point to foresee the global emergence of HIV/AIDS and other so-called emerging infectious diseases and the serious challenges of antibiotic resistance, ecological disruption with its consequences for health and environment (including contributing to emerging diseases) was taking place, and the

devastating impact of infectious disease on the poor and those living in developing or under-developed countries was evident.

• More recently, recognition of the continuing impact of infectious diseases and inadequate nutrition, coupled with ecological disruption, has come with an emphasis on the role of poverty in poorer outcomes and wider disparities.

• Armelagos et al., wrote: “While disease and death are inevitable, a major cause of unnecessary, premature, preventable disease and death is simple; it is extreme poverty.”

3. Another criticism of the traditional theory is that, it fails to distinguish adequately the risk of dying from any given cause or set of causes from the relative contributions of the various causes of death to overall mortality.

• To take an example , accidents (unintentional injuries) are among the leading cause of death among persons, especially those of around 15 to 24 years old in the US, accounting for 46% of all deaths in that age group in 2004, while among those ages 65 to 74 they accounted for just over 2% of all deaths.

• As patterns of disease and mortality change, there are changes in the relative contributions of different causes to overall mortality that may not reflect changes in actual risk.

• Therefore, these and related others are the major contributors of deaths and impairs public health in many modern societies and they are acknowledged as one of the major causes of deaths in our societies.

• Both, the relative contributions and the actual risk of death from the major cause of death categories vary widely across countries, even between countries in the same region, as well as across population groups within a country.

• The point is that, the epidemiologic transition theory oversimplifies the patterns and relations among risk of mortality, mortality causes, and life expectancy.

• The patterns are clearly more complex than simply declining mortality rates from infectious diseases and increasing rates of death from the so-called chronic diseases and do not fit neatly into either historical periods or geographic locations.

4. Also, a further complexity now being recognized is that, the distinction between infectious and chronic disease is not clearly demarcated.

• Not only is it the case that some infectious diseases have chronic disease characteristics, but we have come to recognize the importance of infectious agents and related inflammatory processes in the etiology of a number of chronic diseases and adverse outcomes, such as cervical cancer, gastro duodenal ulcer (H. Pylori), and cardiovascular disease (inflammation), and there is more research now on links to outcomes as divergent as diabetes, preterm delivery, and some mental illnesses.

READ ON:IMPLICATIONS

OF THE THEORY.

POPULATION PROJECTIONSIntroduction• Demographic factors are important components of

both, the causes of and responses to future economic, environmental, and social change.

• Interdisciplinary studies of future global change can draw on projected trends in population size and growth rate, structure, urbanization, and migration, among other variables.

• Often, however, integration does not proceed far beyond uncritical acceptance of a single projection of future population size. For example, studies of environmental change may use projections simply to scale per capita trends in other factors.

• Part of the difficulty in making full use of projections in such work stems from uncertainties is how demographics, acting in concert with social, economic, and cultural forces, may affect the environment.

• However, the historical nature of the projection process has presented obstacles as well. How projections are made, the basis for key assumptions, and how projections differ among institutions that produce them has not always been clear to users, making the interpretation of results somewhat difficult.

• We present a guide to long-term, global projections aimed at researchers and educators who might benefit from putting them to greater use.

• There has been a resurgence of research and new practices in projecting population that are likely to make results more useful and methodology more transparent.

• New thinking is being employed on how best to express uncertainty, on new methodological approaches to projections, and on the likely future courses of fertility and life expectancy.

• In addition, projections have demonstrated the importance of recent revisions to current estimates of demographic variables.

2. Projections and their uses• Population projections differ widely in their

geographic coverage, time horizon, types of output, and use. Spatial dimensions can range from local areas (like counties or cities) to the entire world.

• Local-area projections tend to use shorter time horizons, typically less than 10 years, whereas national and global projections can use decades into the future, and in some cases more than a century.

• These longer-term projections typically produce a more limited number of output variables, primarily population broken down by age and sex.

• In contrast, projections for smaller regions often include other characteristics as well, which might include educational and labour force composition, urban residence, or household type, etc.

• The diversity of types of projections is driven by the diversity of users' needs (Lutz et al 1996a). Commercial organizations often use projections for marketing research and generally want a single most likely forecast.

• They typically want population classified by socioeconomic categories such as income and consumption habits (in addition to age and sex), and by place of residence. Government planners may be concerned with population aging and its potential social and economic impact.

• They may therefore desire longer-term projections, and want to know more about the health status and living arrangements of the elderly.

• The policy community, including advocacy groups, often would like alternatives to a single most likely scenario, including projections that reflect the influence of policy.

• For example, those concerned with the environmental impacts of population growth may be interested in the potential for reductions in such growth through population related policies.

• In addition, they may want to know what the potential effect of environmental feedbacks on growth might be, a topic recently highlighted as underdeveloped by the National Research Council (NRC, 2000).

• Global change researchers often use projections as exogenous inputs to studies on topics such as energy consumption, food supply, and global warming.

• These studies usually require projections with long time horizons (a century or more) and a range of scenarios rather than a single most likely projection.

• We focus here on a relatively small subset of projections: long-term, global population projections - that is, sets of projections that may be made at the national or regional level but that cover the entire world.

• The time horizon of these projections typically ranges from 50 to 150 years.

• Demographers often feel uncomfortable making projections farther than a few decades into the future; uncertainty grows with the time horizon, and increases substantially beyond 30-40 years, when most of the population will be made up of people not yet born.

• Nonetheless, long-term global projections are increasingly in demand by global change researchers and educators. Only a few institutions produce such projections, but research and practice has been evolving rapidly.

3. Who produces projections?• The earliest systematic global population projection

dates to Notestein (1945), although many national level projection efforts began over half a century earlier.

• At our national level, Nations Bureau of Statistics (NBS) has taken a leadership role in the production of projections and dissemination of their results.

• Later efforts, most of which continue to date, have been undertaken by three other institutions :), the IMF and the World Bank (WB), and the International Institute for Applied Systems Analysis (IIASA).

• Global long-run population projections tend not to be undertaken by individual researchers. Individual researchers have tended to create projections at the national-level (or below) and at this level have made significant contributions to varying methodologies.

READ ON : THE OVERALL

SIGNIFICANCE OF POPULATION PROJECTIONS