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The population perspective

1/18/2011

Population perspective

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Principles of Epidemiology for Public Health (EPID600)�

Victor J. Schoenbach, PhD home page

Department of Epidemiology�Gillings School of Global Public Health�University of North Carolina at Chapel Hill

www.unc.edu/epid600/

Habari, Kuna lahiya, Kedu, Konnichiwa, Oseo, Kamunichen, Ni-hao, Bon dia

The health professions that most people interact with are clinically oriented. These professions – medicine, dentistry, nursing, pharmacy, physical therapy, occupational therapy, for example – are primarily focused on individuals. In contrast, epidemiology and other public health professions have as their primary concern the health of populations. This population perspective distinguishes public health from clinical professions. This lecture will describe and illustrate the population perspective. Demography is a science that studies populations, though without a specific focus on population health. Epidemiologists make extensive use of demographic concepts, techniques, and information, as we shall see during the course of this lecture.

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50% x 90% + ?

"Half this game is ninety percent mental." – Philadelphia Phillies manager, Danny Ozark

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I California

"I love California. I practically grew up in Phoenix." – Dan Quayle

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An honest politician

"That lowdown scoundrel deserves to be kicked to death by a jackass, and I'm just the one to do it," – A congressional candidate in Texas.

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Population perspective - outline

1. Epidemiology emphasizes the population perspective

2. Populations are dynamic, diverse, heterogenous - demographic characteristics have major impact on health

a. Fertility and mortality, population pyramids

b. Sex ratio, place, education, gender, ethnicity

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1. Epidemiology emphasizes the population perspective

We see the world through our perspectives.
Individual perspective - focus on health, risk factors, exposures, causal mechanisms in people as individuals
Population perspective - focus on disorders (“mass disease”), exposures, causal mechanisms in people as a group

1/14/2003

Population perspective

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1. Epidemiology emphasizes the population perspective

All of us see the world through our perspectives. My perspective as a faculty member is different from your perspective as a student. Your perspective is different from your parents’ perspective. Primary care physicians have different perspectives than do specialist physicians – and both have very different perspectives than do physicians who review claims on behalf of a managed care organization. Each of us employs numerous perspectives, according to our role and responsibilities in a given activity and our objectives.

In public health we differentiate between two types of perspective – the individual perspective and the population perspective. Epidemiology emphasizes the population perspective.

The individual perspective focuses on the health, risk factors, exposures, and causal mechanisms in people as individuals. Coronary heart disease, cancer, stroke, injury, hypertension, infections, birth defects, asthma, dementia – these are all conditions and events that occur to or in individuals. Each individual has a genetic make-up, early life experience, diet, substance use, physical activity, use of medical care and other characteristics that affect her or his risk for various health outcomes. Preventing and treating health conditions at the individual level is the focus of clinical professions – and also of the public health professions. For example, many epidemiologic studies seek to identify risk factors and causal mechanisms linking those factors to individual health outcomes.

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5/26/2009

Population perspective

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Age-adjusted cancer death rates - US males, 1930-2005

Source: American Cancer Society: www.cancer.org

By contrast, the population perspective focuses on disorders that occur in large numbers of people, sometimes referred to as “mass disease”, and how these disorders are influenced by the physical and social environment. The exposures and causal mechanisms involved are those that function at the level of the group. We can illustrate the population perspective by considering the lung cancer epidemic of the 20^th century in the United States.

The graph in this slide shows age-adjusted death rates from seven types of cancer [colon & rectum, leukemia, liver, lung & bronchus, pancreas, prostate, and stomach] in U.S. males for the years 1930-2005. The graph comes from the American Cancer Society’s publication Cancer Facts & Figures and is based on data from the National Center for Health Statistics and the Bureau of the Census. Notice that these death rates are age-adjusted. We will explain the concepts of age-adjustment and age-standardization later.

What do we see in this graph? For one, we see that death rates for the seven types of cancer have been changing in different ways during the period. At the beginning of the period the death rate from stomach cancer was the highest cancer death rate, about twice as large as the second highest, which was cancer of the colon and rectum. By the end of the period, the stomach cancer death rate was the lowest for these seven cancer sites. Death rates for the other cancer sites changed more modestly, and in different ways – with one exception.

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(Continuation of notes)

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1/6/2002

Population perspective

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5/26/2009

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Age-adjusted cancer death rates - US females, 1930-2005

Source: American Cancer Society: www.cancer.org

Here is the companion graph showing age-adjusted cancer death rates for U.S. females for the same period. This graph also shows the marked decline in stomach cancer death rates during the period. The highest cancer death rate in 1930, however, was that for cancer of the uterus, including the uterine cervix, which also had a marked decline during the period, though not to the same level as stomach cancer. The death rate from cancer of the colon and rectum rose until about 1945 but then began a steady decline to a level substantially below that for males. Breast cancer death rates rose modestly from just below the stomach cancer death rate, becoming the highest death rate between about 1948 and 1986. But again, the most dramatic increase in cancer death rates is that for cancer of the lung and bronchus, which rises from the lowest position in 1930 to be highest by far by the end of the period. The rise in the lung cancer death rate in women comes later than that in men and is not as steep, but it depicts the same phenomenon – an epidemic on a large scale.

It is noteworthy that a major fatal disease could increase to such an extent with so little national outcry – imagine if these deaths were due to West Nile Virus, anthrax, or airplane crashes. Moreover, to the shame of all of us in the health professions – most of the increase in women and about half of that in men occurred after scientists, health professionals, political leaders, and the public knew – or at least should have known – the primary cause of this epidemic.

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(Continuation of notes)

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1/25/2005

Population perspective

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In the words of Jan Stjernsward of WHO, the escalation in tobacco-related deaths has been one of "the most tragic developments in the modern history of medicine" (Jan Stjernsward (WHO), Battle against tobacco. Editorial. Journal of the National Cancer Institute 1989; 81:1524-1525).

In the 1950’s a large number of epidemiologic and other studies tied cigarette smoking to a number of fatal diseases, particularly lung cancer. In 1964 the U.S. Surgeon General issued a landmark report that declared the relation a causal one and subsequently led to mandatory warnings on cigarette packages and advertisements. Television commercials for cigarettes were banned and in the 1970’s and 1980’s a movement grew to restrict smoking in public places. But throughout the entire period cigarettes remained (and remain today) one of the most heavily advertised products, and the tobacco industry was (and is) one of the major lobbying forces in Washington and state capitals throughout the country. The lung cancer epidemic is a vivid demonstration of an epidemic of mass disease and of the need for a population perspective.

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Population perspective vs. the individual perspective on the lung cancer epidemic

Individual perspective concerned with what individual exposures cause lung cancer, lead people to smoke, help people quit, smoking cessation clinics
Population perspective concerned with the environmental factors (including social, economic, technological, political) that promote smoking

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Let’s contrast the individual and population perspectives on the lung cancer epidemic and its control. The individual perspective is concerned with identifying the factors at the individual level that cause lung cancer. Following the demonstration that smoking is a very strong risk factor for lung cancer, the individual perspective examines the factors that lead people to start smoking and help them to quit, and with provision of services to help individuals quit smoking. Smoking cessation clinics, for example, assist people who want to quit to become free of their addiction.

In contrast, the population perspective is concerned with the environmental factors – social, economic, technological, political, etc. – that promote smoking in the population. Tobacco has been smoked by people in the America’s even before Cristóforo Columbo brought the practice back from Cuba in 1493. Tobacco figured prominently in the economics of the American colonies and was used throughout the 19^th century. Yet the lung cancer epidemic did not become evident until the 20^th century.

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A confluence of factors created the conditions for cigarette smoking to mushroom and, along with it, lung cancer. Until late in the nineteenth century, tobacco was used primarily in pipes, cigars, snuff, and chewing forms, mainly because cigarettes had to be hand-rolled (Ravenholt RT. Tobacco's impact on Twentieth-Century U.S. mortality patterns. Am J Prev Med 1985;1(4):4-17). The invention of the cigarette-rolling machine in 1880 revolutionized tobacco use (Ravenholt, 1985). Around this period, the advent of new approaches to curing tobacco produced a variety (“flue-cured”) whose smoke was less irritating. These changes in manufacturing technology, along with the introduction of mass production techniques, improvements in transportation, the growth of the media and the advent of mass marketing (pioneered by the tobacco industry) created the conditions for the explosion of cigarette smoking. Cigarettes’ addictive properties, the glamour of Madison Avenue imagery, and cooperative governmental actions (including distribution of free cigarettes to the armed forces during the First and Second World Wars) helped cigarette smoking to become widespread in the U.S. and the industrialized world (Ravenholt, 1985).

The epidemic probably began early in the century, but its recognition awaited improvements in diagnosis, which according to the late Sir Richard Doll were responsible for much of the lung cancer increase observed during the first half of the 20^th century (Progress against cancer: an epidemiologic assessment. The 1991 John C. Cassel Memorial Lecture. Am J Epidemiol 1991;134:675-).

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HIV pandemic

Individual perspective - counseling and testing, know about HIV, know serostatus, attitudes, safe sex, clean syringes with bleach
Migrant labor patterns, long distance truck driving, subordination of women, globalization, societal response to sex, homosexuality, and drug use

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Another major pandemic* of the 20^th – and apparently the 21^st – centuries is HIV/AIDS (human immunodeficiency virus and acquired immunodeficiency syndrome). The individual perspective on the HIV pandemic focuses on avoiding individual exposure through unprotected sexual intercourse, use of nonsterile drug injection equipment, and exposure to infected blood products. The Centers for Disease Control and Prevention (CDC) has invested millions of dollars in programs to educate, counsel, and test people to teach them about HIV and its transmission, enable them to know whether or not they are infected (i.e., their serostatus), and promote prevention-oriented attitudes and behaviors, such as safer sex and sterilizing of drug injection equipment. [*A pandemic is an epidemic that spreads throughout much of the world.]

The population perspective, by contrast, focuses on the demographic, economic, social, political, and technological factors that have disseminated the virus. HIV probably existed in isolated locations in SubSaharan Africa for many years before the 1970’s. Factors that contributed to worldwide dissemination include (1) migrant labor patterns and long-distance truck driving that take African men far from their villages and keep them apart from their families for prolonged periods; (2) subordination of women in Africa and many other countries, limiting their ability to decline sex and forcing large numbers into commercial sex work; (3) airline travel and globalization, which helped to carry the virus to other continents, (4) changes in sexual mores, (5) and societal responses to sex, homosexuality, and drug use that have obstructed prevention efforts.

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Individual vs. population perspectives’ approach to health problems

Individual perspective - diagnosis (presenting complaint, history, physical exam, lab tests), treatment derived from biomedical understanding of etiology
Population perspective - “community diagnosis” (surveillance, descriptive data, surveys and analytic studies), intervention via health care system, policy, …

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We can also compare the individual and population perspectives in terms of how they approach health problems.

The individual perspective emphasizes diagnosis (based on a patient’s presenting complaint, history, physical examination, and laboratory test results) and treatment derived from the biomedical understanding of the etiology of the disease. Pathology, cellular biology, pathophysiology, microbiology, immunology, and pharmacology are the basic sciences that reveal the etiologic processes within the individual. These sciences underlie diagnosis and treatment, which is applied to people as individuals.

The population perspective employs “community diagnosis”, through collection and analysis of disease surveillance information, descriptive data, surveys and analytic studies) and interventions through changes in the health care system, governmental policy, and environmental changes.

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Population perspective and individual perspective complement each other

Really a continuum - every health condition results from a combination of individual-level factors and population-level factors
Thus, both perspectives are important for public health and clinical medicine, but emphases are different
Political and institutional factors often favor the individual-level perspective

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It is tempting for people in public health to champion the population perspective over the individual perspective, and ride off into the sunset holding the population perspective banner high. However, the two perspectives are really complementary to each other. They also represent two ends of a continuum rather than two discrete alternatives.

Almost every health condition results from a combination of individual-level factors and population-level factors. The former tend to be proximal – they are immediate antecedents of the health outcome. The latter tend to be distal – they often determine the presence of the individual-level factors. But the modes of prevention for mass disease often include individual-level interventions, such as immunizations. Thus, both perspectives are important for and used by public health and clinical medicine. But the emphases are different.

Great advances in the understanding and treatment of disease have been achieved within the individual-level perspective. And, for historical and cultural reasons, the individual-level perspective is more congenial to the U.S. political and institutional framework. There are thus important conceptual, methodological, institutional, and practical advantages for the individual-level perspective. But mass disease – the primary concern of public health – of necessity arises from population-level influences. So the population perspective is fundamental to public health and epidemiology.

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2. Populations are dynamic, diverse, heterogeneous - demographic factors

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Key characteristics:

Size/density, age, sex, place, ethnicity, education, economic resources

Key events and processes:

Birth, marriage, migration, aging, death

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Population Reference Bureau

1/9/2007

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www.prb.org

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My mother works at PRB

“Just wanted to let you know that I'm glad you use PRB reference materials - my mother works there and was actually an editor of the bulletin we used in one of the first lectures. She was thrilled when I told her that you highlighted their work in the lecture. Because they provide many of their publications for a reduced or no cost, it is very important for them to get feedback on which publications are most useful. Please continue to let them know what a good job they do! thanks!”

From Julia, a student in the Certificate Program in Core Public Health Concepts who took the EPID160 Internet course in fall 2005.

9/1/2009

Population perspective

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Population perspective

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*

Population perspective

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From: Joseph A. McFalls, Jr. Population: A lively introduction, 5^th ed, Population Reference Bureau Population Bulletin; 62(1), March 2007.

You are here

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Mortality
Life expectancy
Health status
Fertility

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From: Joseph A. McFalls, Jr. Population: A lively introduction, 5^th ed, Population Reference Bureau Population Bulletin; 62(1), March 2007.

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This table shows life expectancy at birth and at age 65, by sex, in three different years. Life expectancy is essentially a summarization of the mortality rates at each age, such as those presented in the preceding slide. To estimate life expectancy in a given year, we take a hypothetical cohort of people and imagine that they pass through life from birth to death, being subjected to the mortality rates that we are summarizing. So if we began with 100,000 babies, estimate how many would still be alive by their first birthday, then estimate how many of these would still be alive by their second birthday, and so on until the cohort had all died, we could add up the number of years lived by each person in the cohort and divide by 100,000. The result is the life expectancy at birth. So, the female life expectancy at birth in 1900 – 48.3 years, as given in the table – is obtained as the average number of years that each member of a cohort of females would have lived if they experienced the mortality rates recorded in 1900 for each age group. The life expectancy at age 65 is estimated by taking the number of years lived after age 65 and dividing by the number of people who reached age 65.

Note that life expectancy is not a projection or prediction. For the life expectancy in 1900 we use the mortality rates for each age as they were in 1900. But mortality rates have been declining over time, and by the time that someone born in 1900 was age 50, she would experience the mortality rate in 1950 rather than that from 1900. So life expectancy is a way of summarizing the mortality rates across all ages at a specific time, rather than a prediction.

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The life expectancies shown in the table, which come from the National Center for Health Statistics, show that life expectancy increased substantially during the 20^th century, more so for females than for males. Life expectancy at birth was 46.3 and 48.3 years, respectively, for males and females in 1900. By 2000 these figures had become 74.1 and 79.5 years, an increase of about 30 years. The male-female gap in life expectancy, which was two years in 1900, had grown to over 5 years by 2000.

Life expectancy is most heavily influenced by mortality rates at young ages. Improvements in nutrition, hygiene, and conditions of living (housing, child labor), immunizations, and reductions in maternal mortality reduced mortality among infants, children, and women in their reproductive years. These reductions were responsible for much of the increase in life expectancy during the first half of the 20^th century.

Reductions in childhood mortality, of course, have no effect on life expectancy at age 65. Even the elimination of a cause of death such as heart disease can increase life expectancy only modestly, since mortality rates from other causes are so high. Nevertheless during the 20^th century life expectancy at age 65 increased by about 4 years for males and 7 years for females, a substantial gain.

A thought question: life expectancy at birth in 2000 was 76.9 years. Why is this so much less than 65 plus the 17.9 years of life expectancy at age 65?

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6/15/2008

Population perspective

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From: Joseph A. McFalls, Jr. Population: A lively introduction, 5^th ed, Population Reference Bureau Population Bulletin; 62(1), March 2007.

The graph on this slide shows U.S. birth rates by the mother’s age, during the period 1955 to 2002. Again, the data come from the CDC National Center for Health Statistics. Each line on the graph shows the birth rate per 1,000 women for women of different age groups. Birth rates to women in all age groups have come down substantially during the period. Most of the reduction occurred between 1960 and 1975, at which time rates increased for the older age groups.

The peak child-bearing ages are age 20-24 years (the top line) and 25-29 years (the second line). These rates have sharply declined and converged; women in their early twenties are delaying giving birth. The trend toward older mothers is also reflected in the rebound after 1975 in the birth rates for women age 30-34 [the third highest line in 1955] and age 35-39 years [the next to the lowest line, probably mislabeled 34-39].

The rate of births to teenagers age 15-19 years old [the fourth highest line in 1955], an important public health indicator, has declined somewhat during the period. Some of that decline, especially before 1975, may reflect delayed marriage and family formation.

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1/9/2007

Population perspective

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Birth rate calculation

Notes:

1. “Births” means live births

2. For “crude” birth rate, denominator is men and women, all ages. Can also have subgroup birth rates.

3. Often expressed per 1,000 (per year).

We’ve been referring to birth rates, mortality rates, and so forth, without defining them. Let’s make sure that we all understand these important demographic measures.

A birth rate is a ratio of the number of births during a period divided by a reference population. The most commonly-cited birth rates are “crude” birth rates for specific years, calculated as the total number of live births in the year divided by an estimate of the total population at mid-year. The reason for using the mid-year population is that if the population is steadily increasing, as is commonly the case, the mid-year population provides a close approximation to the average number of people in the population on any given day. The use of the mid-year population as a kind of compromise between the starting and ending populations is a common practice for vital statistics rates. Note that the denominator for the crude birth rate consists of both women and men, of all ages, even though most can neither bear children nor father them.

In order to reduce the number of decimal places, birth rates are customarily expressed per 1,000. Although commonly omitted, the time units of the rate (usually “per year”), should be stated, since even though the births were accumulated during one year, the number itself can be expressed “per month”, “per week”, “per decade”, or with respect to any other time unit.

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9/30/2008

Population perspective

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Fertility rate calculation

Notes:

1. Denominator is women of reproductive age (15-44 or 49 years) (per unit time).

2. Often expressed per 1,000 (per year)

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9/30/2008

Population perspective

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Death rate calculation

Notes:

1. Denominator is total population (men and women, all ages) (or can have subgroup death rates) [/ time]

2. Often expressed per 1,000 people (/year)

You’ve probably already guessed that an annual death (or mortality) rate is estimated as the number of deaths divided by the mid-year population per year and is often expressed per 1,000 people. Again, “per year” is generally intended but is stated only infrequently. As with the crude birth rate, the denominator for the crude death rate includes the total population (men, women, and all ages), even though mortality rates vary greatly by age and gender. For this reason, the overall death rate is not used very often. More typically death rates are calculated for specific subgroups defined by gender and age. When an overall figure is desired, the rates for the different age groups are combined as a weighted average – the resulting average is called an “age-adjusted” or “age-standardized” mortality rate and is typically computed separately for males and females.

As we have seen in earlier slides, death rates may be estimated for specific causes of death, such as all cancers, specific cancer sites, heart disease, and so on. A major concern in interpreting all vital statistics and epidemiologic rates is the quality of the data that were available to calculate them. In developed countries, birth records, death records, and population size estimates are generally available and accurate. Information on cause of death, however, is much less reliable, since there are various definitional and practical obstacles in accurately classifying deaths as to cause. Thus, before relying on data on cause-specific mortality, it is important to assess the likely accuracy for each specific cause of death being analyzed. More problematic is the situation in less-developed countries, where population counts and vital events registration may be incomplete to varying degrees.

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9/30/2008

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Average annual death rate calculation

Notes:

1. Denominator is total population (men and women, all ages) [/ time]

2. Often expressed per 1,000 people (/year)

When a rate is estimated from a small number of events, the resulting estimate is unstable. If even a few events were miscounted, or if there were a minor variation that particular year, the rate could be misleading. Estimates based on small numbers are called “imprecise” or “unreliable”, and generally avoided.

A common strategy, therefore, is to estimate an average over several years, as a way of obtaining a larger number of events for the calculation. The slide above illustrates the calculation of an average annual death rate. Here the average number of deaths during the n year period is used in the numerator. The denominator estimates the average size of the population (as usual, by taking the size of the population at the midpoint). So for calculating the 3-year average annual death rate for 1999-2001, the numerator would contain all deaths during 1999-2001, divided by 3, and the denominator would be the population estimate for July 1, 2000. Since the year in which the Census count takes place has the most nearly accurate population size estimate, the period is usually chosen so that the Census year is in the middle.

Once again, as with the other rates we have seen, the denominator actually includes the time interval during which the deaths have accumulated, and therefore should include the units in which that time interval is expressed (e.g., years). Although the units are typically omitted when vital statistics rates are presented, they are in fact ambiguous without their units, usually “per year”.

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Age structure
Population growth
Fertility, momentum
Demographic transition
Population aging

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Population pyramids

From: Joseph A. McFalls, Jr. Population: A lively introduction, 5^th ed, Population Reference Bureau Population Bulletin; 62(1), March 2007.

The age-sex make-up of a population is often displayed in a “population pyramid”. A population pyramid is constructed by drawing a series of horizontal bars stacked one on top of the other. The bars represent age strata and generally have the same height (e.g., representing 5-year age bands). The width of the bars is drawn to correspond to the proportion of all males (on the left) and all females (on the right) who fall into that age band. Since age and gender are such fundamental characteristics in many areas of life, including health, the age and gender structure of a population, as presented in its population pyramid, tells us a lot about the population.

The graph in this slide shows population pyramids for three different countries, illustrating three basic population structures. On the left, the pyramid for Kenya depicts a rapidly-growing population. How can we see that from the pyramid? The narrow middle and top and widening base indicate that each new birth cohort is larger than the previous one. With the passage of time, each age band moves up the pyramid, so these larger cohorts are expanding the total population. High adult mortality rates help to narrow the pyramid at the top.

The center pyramid shows the U.S., a slow-growth population. The bands at the base are narrower than those in the middle, indicating a lower fertility rate. The bulge in the middle corresponds to birth years 1945-1965, approximately the “baby boom”. The pyramid for Denmark, on the right, depicts zero growth.

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A record of a population’s history

From: Joseph A. McFalls, Jr. Population: A lively introduction, 5^th ed, Population Reference Bureau Population Bulletin; 62(1), March 2007.

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Comparing population pyramids over time shows how rising and falling birth rates and population aging influence the age-sex distribution over time. This slide shows the U.S. population by age and sex, in 1900, 1970, 2000, and projected for 2030. The graph for 1900 – in the upper left of the slide – actually looks like a pyramid, as did the one for Ethiopia. The slope of the sides of the U.S. pyramid is less steep, though, corresponding to a lower birthrate and perhaps higher infant mortality. The U.S. in 1900 was growing from immigration as well as births, which also makes for a broader pyramid.

The pyramid in the upper right (1970) looks markedly different, almost as if the 1900 distribution had had a girdle placed around the 30-39 year age band, so that the younger population had not been allowed to grow up. The narrowing in the age groups from ages 25-39 reflects the decline in fertility during the Great Depression of the 1930s and World War II. Children born in 1944 were about 26 in 1970 (see my picture?), those born in 1939 were 31 in 1970, and those born in 1930 were 40. The younger age bands – ages 5 through 24 – show the Baby Boom. In this way, a population pyramid displays the reproductive history of a population. Reflecting on the shape of the 1970 pyramid, with its substantially greater ratio of youth to parents, it’s no wonder that the 1960s and 70s were such a transforming time in the U.S.

Coming closer to home, chronologically speaking, the lower left pyramid for the year 2000 shows the Boomers at maturity and the “Boomlet” of their offspring. In the spirit of “Where’s Waldo”, find my picture next to the caption “Men”. By 2030, I predict that funding for the NIA will have eclipsed that for NICHD.

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From: Carl Haub and O.P. Sharma. India’s Population Reality: Reconciling Change and Tradition, Population Reference Bureau Population Bulletin; 61(3), Sept 2006.

Moving to the other side of the globe, this graph shows population growth in India during the 200 years from 1801-2001. The second country to pass the one-billion population mark, India is home to about one-sixth of the world’s population. The population growth curve mirrors that for the world as a whole. Each year there are more women in their reproductive years and, even with the same fertility rate, there are more babies. If the curve were redrawn to show the percentage increase in population each year (e.g., by plotting the data on a logarithmic scale), the decline in the growth rate would be visible.

________

[Note for previous slide: NIA = the National Institute on Aging (of the U.S. National Institutes of Health); NICHD = the National Institute of Child and Human Development.]

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From: Joseph A. McFalls, Jr. Population: A lively introduction, 5^th ed, Population Reference Bureau Population Bulletin; 62(1), March 2007.

The demographic transition is a general pattern of changes in death rates, population growth, and birth rates that appears during the process of modernization. Demographers distinguish four stages. Stage 1 is the period before the transition begins. Birth rates are high, but so are death rates, so population size is stable. The transition period begins when death rates fall, as improvements in agricultural productivity, economic development, and introduction of modern sanitation, immunization, improved nutrition, and other public health measures become available. However, cultural values, considerations of personal security, personal preferences, and the priorities of the health professions continue to support the high birth rate. The population begins to grow rapidly.

The transition from stage 2 to stage 3 begins when, under the influence of such factors as the effects of land shortages, urbanization, education of women, and introduction of family planning programs, the birth rate begins to decline. In stage 3, the decline in the birth rate exceeds that in the death rate, which comes to stabilize at a low level. Population growth continues, but at a declining rate.

In stage 4, the transition has been completed, as the birth rate joins the death rate at a stable, low rate, and population size stabilizes. The lower birth rate can produce a “demographic dividend” from fewer dependents per working-age adult, giving a country the opportunity to invest in health care, education, economic development, and infrastructure before the population ages.

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This slide shows fertility rates in India that were responsible for the decrease in the birth rate shown in the earlier slide. The graph shows the total fertility rate (TFR), which is a summary of age-specific fertility rates and will be explained in a later slide. Roughly speaking, the TFR is the number of babies that the average woman will have during her reproductive lifetime. A TFR of two is approximately replacement level, since the two births to the average woman will take the place of herself and her mate (hypothetically only one per woman).

In the graph, we see that the TFR has declined more-or-less steadily since 1970 in women in both rural and urban areas. In general, fertility is lower in urban than in rural areas. Indian women in urban areas have a TFR of between 2 and 3, not too far above replacement level. Indian women in rural areas, however, have a much higher TFR and contribute substantially to India’s population growth.

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This next slide shows the decline in Chinese fertility during approximately the same period (1950-1990), again in terms of the TFR. In the second half of the 20^th century, China underwent one of the fastest demographic transitions in history (Tien HY et al., PRB 1992). The TFR dropped dramatically from 6 for rural women and 5 for urban women to about 3 for rural women and less than 2.0 for urban women. The decline was steepest in the late 1950’s and early 1960’s, the period of Mao Zedong’s Great Leap Forward (1958-1960) – an attempt to accelerate economic progress by introducing a new socioeconomic and political system based on “people’s communes” of about 5,000 families each. The Great Leap Forward led to widespread famine, with sharply higher mortality and lower fertility. TFR rebounded after the Great Leap, but has turned down steadily since about 1970 as a consequence of China’s one-child policy.

H. Yuan Tien, with Zhang Tianlu, Ping Yu, Li Jingneng, Liang Zhongtang. China’s demographic dilemmas. Population Bulletin 1992 (June): 47(1)

___________

You can read about the Great Leap Forward at these two sites:

http://www-chaos.umd.edu/history/prc2.html#greatleap

http://www.mtholyoke.edu/courses/sgabriel/economics/china-essays/4.html

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Total fertility rate

Summarizes current fertility rates by imagining a cohort of women moving through their reproductive age years

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15 20 25 30 35 40 45

Let’s pause here to explain the concept of the total fertility rate (TFR). The TFR is a summary measure, constructed in a manner reminiscent of life expectancy. For biological and social reasons, fertility (the number of babies divided by the number of women in their reproductive years) varies greatly by age. If we want to characterize fertility with a single number, we need a way to summarize the overall picture. The measure that is typically used for this purpose is the TFR, and it can be regarded simply as the sum of the age-specific fertility rates for each year of age between 15 years and 44 (or 49) years, inclusive. TFR is typically described as the number of births expected per woman for an imaginary a cohort of women moving through their reproductive lifetimes (generally taken as ages 15-44 or 15-49 years). The expectation is based on the current fertility rates at each age in a given year. The next slide shows an example of such a calculation.

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Total fertility rate calculation

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Total fertility rate calculation

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In this table the fertility rates are shown per 1,000 women per year.

If these age-specific fertility rates were to stay that way for the next 30 years, a cohort of 1,000 women age 15 years old today would be expected to have about 104 babies at age 15, 104 babies at age 16, 104 babies at age 17, 104 babies at age 18, and 104 babies at age 19, or a total of 5 × 104 = 520 babies by the time the women reached their 20^th birthday. As these women moved through the five years from age 20 to 24 years, the women would have another 136 babies each year, or 5 × 136 = 680 babies by the women’s 25^th birthday.

During their next five years (ages 25-29 years of age) the women would have 5 × 128 = 640 babies, and then 570, 165, and 95 babies, respectively, during the next three 5-year intervals. Adding these numbers up, we get 2,670 babies born to the 1,000 women, or 2.67 babies per woman, giving us the total fertility rate (if you calculate without rounding the intermediate results you will obtain 2.68, which is more nearly precise).

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Total fertility rate calculation

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Population perspective

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The table on this slide shows the same data, but the fertility rates are expressed per woman per year instead of per 1,000 women per year. The expected births are expressed per woman.

So the TFR of 2.67 tells us the number of births that the average woman would have during her reproductive lifespan if she experienced the above age-specific fertility rates during her reproductive lifetime. Note that the TFR is a summary, not a prediction, since we did not attempt to forecast age-specific fertility rates into the future. We imagined the women moving through their reproductive years, but we kept the age-specific fertility rates fixed as of a given calendar year or interval. For a forecast, such as would be used to project population growth into the future, we would need to consider whether factors such as urbanization, greater education of women, and advances in contraceptive technology would reduce age-specific fertility rates below their current levels.

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China’s Age Distribution by age and sex, 1964, 1982, and 2000

From Figure 6. China’s Population by Age and Sex, 1964, 1982, and 2000 from Nancy E. Riley, China’s Population: New trends and challenges. Population Bulletin 2004: 59(2);21.

Original sources: Census Bureau, International Data Base (www.census.gov/ipc/www/idbnew.html, accessed April 7, 2004); and tabulations from the China 2000 Census.

6/19/2009

This slide shows three population pyramids for China. The constrictions at ages 15-24 years in 1964, which are echoed in ages 35-44 year in 1982 likely reflect the impact of World War II and the Japanese occupation. The 20-24-year-olds in 1982, whose numbers are severely curtailed, consists of people born during “The Great Leap Forward”, described on the slide about fertility in China.

The widening of the 1982 pyramid at younger ages is partly due to a catch-up phenomenon of families having the babies which they would have had during 1958-1962. In the 2000 pyramid we see the “echos,” one generation later, of the constriction from the Great Leap Forward and of the subsequent rebound.

Meanwhile the 1982 and 2000 pyramids, with their narrow bases, show the impact of China’s successful though in some respects controversial population policies and the aging of the population that is occurring.

_________

For more on China’s the demography in China during the 20^th century, see:

Demographic Consequences of the Great Leap Forward in China's Provinces�Xizhe Peng, Population and Development Review 1987(Dec);13(4): 639-670, www.jstor.org/stable/1973026

Demographic Dimensions of China's Development, Eduard B. Vermeer, pp. 115-144, www.jstor.org/stable/20058946 in Population and Development Review, 2006;32. The Political Economy of Global Population Change, 1950-2050, www.jstor.org/stable/i20058940

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7/1/2009

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From: Nancy E. Riley.. China’s population: new trends and challenges. Population Reference Bureau Population Bulletin; 59(2), June 2004.

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1/27/2002

Population perspective

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This slide shows a series of stacked bars with the age distribution of older Americans (age 65 years and above) from 1900-2000, and projected to 2050. The graph indicates how the age distribution among seniors has moved up the age ladder. In 1900, most seniors (65 years and older) were younger than age 75, and only 5% were 85+ years. By 2050, “young seniors” (age 65-74) will be a minority of all seniors, while nearly one-in-four will be 85+.

The “graying” of American society has been underway since the middle of the 20^th century and is transforming the nature of society in many respects. Older people vote differently from younger people, older people have different health conditions and concerns, older people have more expensive health problems than younger people (e.g., one-in-four persons age 80 and older have Alzheimer’s disease), and older people for the most part are living on retirement assets and government programs.

The dependency ratio is the number of children and adults over 65 to the number of people of working age, who provide much of the financial and instrumental support for the young and the old. As the population ages, the dependency ratio will increase. Even if the retired population has substantial financial assets on which to live, it still takes people to provide the services they need. We can anticipate shortages even more severe then the current nursing shortage unless we greatly improve the educational preparation of young people and/or increase immigration.

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Population perspective

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Chronic conditions among older Americans

This slide illustrates the high prevalence of chronic medical conditions among Americans age 65 and above as well as substantial differences across three ethnic groups. Ethnicity refers to groups of people with shared culture, language, religion, and/or geographical origin. Many countries have multiple ethnic groups. In the United States the the term “race” is often used to classify people based on skin color and several other physical features. In the United States that distinction has been used as a basis for enslaving millions of African Americans, killing or dispossessing American Indians, and discriminating against Asians and others. A biological rationale for these actions was constructed on the foundation that people belonging to different “races” were fundamentally different. This view has been largely discredited, but race remains very much a reality because of its connection to social, economic, and political factors as well as cultural and historical experience. Disparities in health status for different ethnic groups are a major public health concern.

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1/27/2002

Population perspective

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Sex ratio

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1/18/2011

Population perspective

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From: Joseph A. McFalls, Jr. Population: A lively introduction, 5^th ed, Population Reference Bureau Population Bulletin; 62(1), March 2007.

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9/8/2005

Population perspective

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1985

1980

1975

1970

106

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110

112

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104

102

100

Figure 5�Sex Ratios at Birth, 1970-1987

Source: Sten Johansson and Ola Nygren, “The Missing Girls of China: A New Demographic Account,” Population and Development Review Vol. 17, No. 7 (March 1991): Table 1, p. 39�Graph from Population Reference Bureau.

Normal sex ratio at birth

Males / 100 females

This slide shows sex ratios at birth, from 1970-1987, in China. Sex ratios in most periods and most countries we have data for are generally 105-106 (per 100 females) at birth and fall gradually with age. In the 1964 census in China it was 103.8 at birth and 105.3 at one year of age (Johansson and Nygren, 1991).

In this graph we see that the sex ratio at birth rose in the mid-1980s, rising to an exceptionally high level. What is happening here? Several explanations have been considered. China introduced a one-child policy in 1979, and there is a strong cultural preference for males. One effect was underreporting of female births so they would not count against the one-child quota per couple. Many of the unreported girls are apparently given away for adoption (Johansson and Nygren, 1991). Another part of the explanation may be higher mortality for female infants who are not reported, due to deliberate infanticide or relative neglect of female infants. The potential exists for a lot of Chinese men who will not be able to find Chinese women.

[See Johansson S, Nygren O. The Missing Girls of China: A New Demographic Account.�Population and Development Review, 1991; 17(1): 35-51. (available through UNC libraries e-journals)

See also www.prb.org/Content/ContentGroups/Datasheets/Women_2002_Demography_Overview.htm

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Place�Education�Status of women

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Population perspective

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From: Joseph A. McFalls, Jr. Population: A lively introduction, 5^th ed, Population Reference Bureau Population Bulletin; 62(1), March 2007.

Along with the expansion of the world’s population during the second half of the 20^th century has come a dramatic shift in its global distribution. Asia and the Pacific, where a majority of the world’s population live, have increased their dominance though their share is projected to decline modestly by mid-21^st century. Europe’s share of world population declined from 25% to 14% from 1950 to 2005 and is projected to decline to 9% by mid-21^st century. Meanwhile, in spite of the AIDS epidemic Africa’s share is projected to increase to 21% from 9% in 1950 and 14% in 2005. The Americas’ share of world population has remained stable at 14%, but with a shift from Northern America to Latin America. In addition to the changes shown in the figure, the populations of Europe and especially Northern America are increasingly composed of immigrants from Africa, Latin America, and Asia.

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1/9/2007

Population perspective

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This slide shows population and demographic rates for the 10 most populous countries in 1995. China is the largest, with over 1 billion people. India is second, with 930 million (these are 1995 data; India is now above one billion.

There is wide diversity across these ten most populous countries, in population, population density, birth rates, death rates, infant mortality, natural increase (birth rate – death rate), total fertility rate, infant mortality, life expectancy, and percent urban residents.

Although it is a highly industrialized country, the U.S. has a birth rate that is only slightly lower than China’s, and considerably greater than Japan’s. One factor that boosts the U.S. birth rate is the growing proportion of Latinos, who tend to be younger and to have larger families. The birth and death rates in this table are not age-standardized, so they reflect the population age distributions. The U.S. death rate is greater than China’s, partly for that reason.

Indonesia with nearly 200 million in 1995 had a birth rate that was intermediate between those of China and India, a death rate that was lower than India’s, a rate of natural increase that was lower than India’s but higher than China’s. Life expectancy that was on the low side for males. It is also only 31% urban.

This is an example of the kinds of things one can see about countries by examining their data, and the wide diversity across nations. The Census Bureau’s International Data Base (IDB) is a very convenient source for this kind of demographic information.

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This slide shows the World’s largest megaticities in 1970 and projected for 2015. In 1970, Tokyo, New York, Shanghai, Osaka, and Mexico were the five largest cities, followed by London, Paris, Buenos Aires, Los Angeles, and Beijing. In 2015, by contrast, the largest cities will be Mumbai (also known as Bombay), Tokyo, Lagos, Dhaka, Sao Paolo, Karachi, Mexico City, Delhi, New York, and Jakarta. All but two of the largest cities in 2015 will be in developing countries. At present, cities in developing countries often lack essential infrastructure, such as safe drinking water, modern sanitation, adequate housing, etc. According to a 1996 WHO fact sheet�(www.who.int/inf-fs/en/fact122.html), 300 million urban residents had no proper water supply in 1994, and 600 million residents had no sanitation facilities. Although the proportion of urban dwellers served by piped water and sanitation had improved during the preceding 20 years, growth of the urban population increased the absolute number of people not served.

[See also:�www.prb.org/Content/NavigationMenu/PRB/AboutPRB/Population_Bulletin2/An_Urbanizing_World.htm�]

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1/27/2002

Population perspective

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Ethnicity

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This graph shows the prevalence – that is, the percentage of the population who are afflicted – of six chronic conditions [arthritis, hypertension, heart disease, cancer, diabetes, and stroke] for two age groups: 70-74 years and 85+ years in 1995. Most of these conditions are more prevalent in the older age group, but what is perhaps most significant in the graph is the substantial prevalences of older Americans who have arthritis (over half), hypertension (nearly half), heart disease or a history of cancer (about one-in-five for each), diabetes and history of stroke (about one-in-ten for each).

Since the population of elderly is expanding rapidly, it is likely that we will see much larger numbers (though not necessarily larger percentages) of older people with chronic health conditions. We cannot automatically conclude that when today’s 50-75-year-olds are 70-95 they will experience chronic health conditions at these percentages, since people who in 1995 were older than age 70 years were born before 1925, and therefore had different life experiences and exposures than people who will pass age 70 during the next two decades. But it is likely that the prevalence of these conditions will be at least as great, since prevention of arthritis and hypertension is not on the horizon, whereas survival from heart disease and cancer will probably increase. The obesity epidemic will probably contribute to higher rates of diabetes and probably heart disease and cancer – if survival improves, the prevalence of these conditions will increase.

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1/9/2007

Population perspective

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The United States more than most large countries is a nation of immigrants. The first immigrants were Amerindians, probably from Asia, who arrived on the continent 10,000-30,000 years ago, probably in multiple waves. Europeans arrived in the 16^th century and during the course of the next few hundred years took most of the land from the American Indians. The Europeans imported millions of Africans to work as slaves and promoted immigration from northern Europe, especially, to populate the country. Changes in world demography, economic conditions, and political factors have produced major shifts in immigration throughout U.S. history. This slides shows that at mid-century most immigrants still came from Europe, with a small percentage from Asia. Thirty years later the share of immigrants from Asia, previously restricted by U.S. immigration policies, increased greatly. (In December 2006 the Population Reference Bureau published a report “Immigration: Shaping and Reshaping America”)

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Population perspective

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Key points

Think populations

Populations are diverse

Populations are heterogenous

Populations change

Demographic indicators and graphs attempt to observe populations

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The right to bear arms

Hunter's rifle goes off accidentally at airport

GRAPEVINE, Texas — A deer hunter's rifle accidentally went off at the Dallas-Fort Worth Airport on Friday as the owner tried to show a ticket agent that the weapon was not loaded. No one was hurt.

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The bullet went through a window and landed in a planter.

Airport spokeswoman Tina Sharp said the incident happened as the gun's owner was checking luggage at a Delta ticket counter for an elk-hunting trip.

No charges were filed against the man. He and his wife were allowed to board a later flight. Airport operations were not interrupted.

Durham Herald-Sun, 11/24/2001