Lecture 1-

Approaches used to structure teaching of introductory population courses.

There are three approaches that are typically used in many courses in population problems, introduction to population, or introduction to demography.
The first is reading in popular ecological literature without defining demographic terms or elaborating relevant conceptual frameworks. This approach often leaves the student with little, other than a vague awareness of the rhetoric on the relationship between the population pressure and environmental issues and a somewhat confused understanding of technical information in demography.
The second approach is closely related. It uses science fiction. This approach, while interesting and intellectually titillating, often limits its analysis to the assumptions of a particular framework, but does not explore the nature of research that supports that framework.
The third approach is traditional, and by far the most popular approach. It is develops demographic terms and related frameworks, then discusses population’s impact on social institutions and vice versa. This approach provides a foundation for more advanced courses in the field and preparation for reading and comprehending much of the professional literature.

Definition of demography

Demography is the systematic study of population. Demographers typically focus on two aspects of population: its structure and its dynamics of change.
The structural aspects include: size, number of individuals; composition, distribution of individuals among age, sex, income, occupation, educational groupings; and so on; and geographical distribution, people’s dispersion and density. The dynamics of change include the three processes of fertility, mortality, and migration. Some demographers also considered in nuptiality (marriage) and social mobility (changes in social status) among the dynamics of change. Hauser and Duncan 1959:2, Bogue 1969:1-4, Champion et al, Sociology, 1984: 386-387.)

Independent and dependent variables

One objective of the study of population is to create a statement or set of statements which relate one phenomenon to another or a set of phenomenon to another set. This exercise is called the creation of theory. Theory is comprised as of a set of statements that describe, or explain how phenomena are interrelated, or why they behave in the manner they do. The usual statement format specifies some phenomenon or condition that influences, shapes, precedes, or causes a second condition. The first condition may be called variously cause, predictor, or independent variable. The second condition may be called effect, predictand, or dependent variable. Before the age of the computer and the compilation of myriad, large data sets the object was to establish single causal statements. Now multiple causal statements are the backbone in theory and model building and testing.

If statements of relationship are not verified, there are called hypotheses. Those statements that have been verified are called empirical generalizations. Debate long centered over whether or not the population analysts employed theory, whether the primary variables should be treated as independent or dependent variables in research studies. The debate also included the issue of whether or not interdisciplinary research should be included or excluded from the demographers’ domain of investigation. Demographers employ heuristic theory and multiple causal models, use population variables as independent and dependent variables, and interdisciplinary research abounds. The study of population is not restricted to formal analyses of the interrelationships between births, deaths, and migration in light of the above assumptions. It also includes the investigation of interrelationships between various social characteristics and fertility, mortality, and migration. It asserts that population change is socially determined as well as socially determining. Investigators employing this perspective are drawn from a wide range of disciplines. For instance, economists, sociologists, statisticians, public health officials, medical doctors, biologists, geographers, historians, and geneticists have conducted research using demographic techniques or looking at problems involving population variables.

Variables of demography

The primary variables of concerns are those specified in the demographic equation (see below), births, deaths, and migration. The primary variables are those biological events/processes that contribute to change in population size either through replacement or loss.
Fertility is the actual performance of childbearing. It is sometimes called natality in the professional literature. Fertility is not to be confused with fecundity.
Fecundity is the biological capacity to bear children.

Mortality is death. For our purposes only the biological definitions of death concern us. We do not investigate the concept of social death. Mortality is not to be confused with morbidity. Morbidity deals with sickness.

Migration is the movement of individuals across some politically defined territorial boundary. Residence may or may not be required according to the definition used. In some cases all that is required to qualifying as a migrant is the intent to change place of residence.

By substituting the appropriate figures in the demographic equation for numbers of births, deaths, and migration, we can calculate the rate of natural increase or decrease in a population or calculate the amount of net migration, as well as change in population size.

In addition to these variables demographers we also examine geographic mobility, social mobility, and nuptiality. Geographic mobility includes the dispersion and distribution of individuals and activities across geographical and statistical areal units. Social mobility examines the movement of individuals between the social categories. For instance, individuals who moved from unemployed to employed, employed to retired, or who moved from high school to college, or college to graduate school. Nuptiality examines transformations from single to married, married to divorced, divorced to re-married, married to widowed, or widowed to remarried.

Secondary variables are those defined in a perspective other than demography. These variables may be measured using indices with population data or individual level data. Secondary variables specify properties of individuals that indirectly reflect status characteristics or positions in the social order. They are often examined by themselves or correlated with the primary variables. We assume that these indices indirectly measure various dimensions of specific social institutions or various ongoing processes of the social order. Further we assume that a combination of social indicators and demographic indicators allow us to interpret, describe, explain or predict regularities in the population composition in the organization of day-to-day activities of the social order. Further, we assume that such an approach allows us to evaluate change in these regularities either with any particular population/society or between populations and societies.

Population composition was mentioned above without specification. To the demographer, composition means that social or biological traits can be measured in or treated in a distributive matter. The prime indicators for the demographer of population composition are age and sex. The demographic literature refers to this age-sex distribution as population structure. Both sociologists and demographers in focusing on age and sex assume that all societies specify role demands using age and/or sex as criteria for grading activities. Age usually is tapped "as of the last birthday" and then standardized from the date of survey or enumeration.

To the sociologists, composition means the distribution of individuals in a wide range of statuses found in the social order. For the sociologist, status definitions are reflective of the composition of the population and the social order. Some statuses are fairly stable and long-term assignments. Such statuses are ascribed at birth. Other statuses are transient and subject to rapid modification. Such statuses are achieved. Our knowledge of these expectations of what an individual is suppose to do, allows us to look at the distribution of individuals in a variety of statuses and imagine what kinds of social structures obtain and processes may occur. Examples of important statuses that demographers examine include education, ethnicity, race, family background, income, occupation, employment by type of industry.

Tertiary variables are time and space, implicitly or explicitly referenced in every investigation. Tertiary variables locate a population in space and history. The spatial unit ranges from blocks to enumeration districts, census tracts, cities, and towns, townships, incorporated places, counties, parishes, states/provinces, regions and countries of the world. Time usually refers to the date or dates that population data were gathered. The size of the spatial unit often is used as our criterion for distinguishing between Micro and macro demographic investigations. Note that this distinction sometimes is very arbitrary. Demographic studies which focus on the distribution of activities in space are referred to as human ecology studies and are identified with the sociology apartment at the University of Chicago during the 1920s and 30s. Later in the course when we investigate urbanization we will describe some of the major patterns that typify how human activities are arranged in space and time.

Add comments about spatial, geographical, statistical, political areal units serve as surrogate indicators of community.

Total population

With a variety of statistical resources available, students of population should be cognizant of different definitions of population coverage in various definitions of several administrative units as well as changes historically in such definitions. Such knowledge is useful to assess the adequacy of population coverage and establish the comparability of statistical materials when obtaining information on such a simple fact as size.

Demographers traditionally have designated two types of total population. De facto, that is, all persons residing in a specified area at the time of observation, and de jure, that is, all persons along and, having legal residence or usual residence, in a specified area at the time of observation. De facto and de jure population designations are pertinent for scholars of jurisprudence, and for students of population when trying to figure out what enumerators and canvassers are doing in assigning individuals to a particular locale. The United Nations guidelines for census administration specify the criteria for inclusion and exclusion of individuals must be clearly defined to ensure accurate coverage of the following categories of people: autochthonous inhabitants and nomadic tribes; military, naval, and diplomatic personnel and their families located abroad; merchants seem in resident in the country but see if the time of the census; other civilian national residence temporarily abroad at the time of the census; foreign military, naval, and diplomatic personnel in their families located on the country; and other civilian aliens temporarily in the country of the time of residence excuse me at the time of census.

Historically, has the United States census has enumerated populations employing the criteria of usual residence to allocate individuals to households or geographic locales. Such a practice is rooted in the constitutional requirements for legislative representation and apportionment, for allocation of federal funds to the states, and state funds to municipalities, and the research tradition focusing on areal units in constructing social indicators of “well-being” for planning purposes. Students using census materials for government sources predicated on census data sheet should consult the Numerator’s Reference Manual for definitions employed in including then excluding individuals of specific characteristics and usual place of residence. Special censuses, current population surveys, and national sample surveys typically follow the concept of usual residence. State reports based on registration of vital events seek to ascertain whether information is referenced by residence or place of occurrence.

Assumptions about population (For a more detailed discussion of this set of assumptions see: Norman B. Ryder. “Notes on the concept of population.” American Journal of Sociology 69, 5 (March 1964): 447–463.)

We assume the population is an aggregate or a set of individuals. This aggregate exists in space and persists overtime. We assume that individuals are substitutable. We assume that statistics on aggregate data can be used to depict the statistical hypothetical average individual. These statements often appear representing aggregate findings, but these are couched in language referencing the typical individual. For instance, what is the average number of days in the menstrual cycle and what are the characteristics of the typical migrant. Comments on the aggregate fallacy.

We assume the additivity of individuals and their experiences to yield the aggregate experience. Presupposition of interaction is often taken as a given, although not tested directly. For the behavioral scientist there are different notions of shared. There is the notion of the aggregate in a statistical sense. People with a common characteristic may interact or have a similar set of life experiences and chances. There is the notion of interaction in which people recognize formally defined rules and understandings about what each and every individual is going to do to accomplish group goals or to fulfill their obligations to other members of the group. In demography these assumptions are not tested directly, but rather we assume that the aggregate characteristic of a group may in fact be a prerequisite for the possibility of interaction among individuals or who have like life experiences or chances. Structural homology.

Demographic equation

The investigation of human numbers attempts to summarize available data on the size and composition (biological and social characteristics) of the population and mathematically describe changes in these population components. Changes in the size of the aggregate are result of additions to the aggregate, that it, births and in-migration and losses from the aggregate, that is, deaths and out migration. Change explicitly involves a time dimension, hence additions and subtractions in the aggregate allow us to describe shifts in population size through time.

The basic formula expressing these relationships is the demographic equation or the balancing equation or the accounting equation.This equation is defined:

P² = P¹ + (B - D) + (I - E) + e

B = Sum of births from T^1->2period of observation using vital statistics data

D = Sum of deaths from T^1->2 period of observation using vital statistics data

I = Sum of immigrants from T^1->2  period of observation may be available from immigration records.

E = Sum of emigrants from T^1->2 period of observation may be available from immigration records.

e = measurement error

P² = Population size based on census at end of period of observation

P¹ = Population size based on census at beginning of period of observation

or restated as:

P² = P¹ + natural increase + net migration + e

Of the two components in accounting for overpopulation and growth the most important is natural increase. Natural increase is defined as births minus deaths. People have the biological capacity to reproduce many times, but they only die once. Net migration is defined as in migration minus out migration.

Another way of looking at population figures is to combine registration statistics on vital events and information from the census statistics to calculate crude rates. The crude rate is defined the number of the events divided by the population at risk times a constant.

CR= ( _nE_x /_nP_x) * k , where n= no of cases and x=age.

The k (constant) may be 1 for probabilities, 100 for percentages, 1,000 for rates, and where there are large numbers of events, 100,000 for rare events.

In general crude rates provide information that pertains to the entire population. Comparison of crude rates gives a demographer a rough idea about which country has the highest or the lowest birth rate, death rate, or rate of natural increase.

CBR=56 high CBR=10 low; CDR=55 high CDR=under 10 low. See historical demography for discussion of high and low IFMRs.

The rate of natural increase is calculated by subtracting in the crude death rate from the crude birth rate. The crude birth rate and the crude death rate are calculated to the base 1000. If the crude birth rate equals 17.6 and the crude death rate equals 9.6, then the crude rate of net natural increase rate equals 8.0. As calculated, the rate of natural increase is expressed to the base 1000. It is customary to express the rate of natural increase as a percent per annum, thus in this case the rate of natural increase would be .8 percent per year. All you need to do is to shift the decimal place one place to the left to obtain the percent of increase per year.

The rate of natural increase is used to calculate how fast the population size will double itself. To calculate the doubling time for the population: divide 70 by the percentage of natural increase per year, the result is the number of years for the population to double in size. This rule is based on a simple exponential function. A natural increase of .5 % per annum will yield 139 years, 1 % will yield 70 years, 1.5 % will yield 47 years and so forth 4.0% percent per annum will see a population double itself in 18 years. If you wish to develop an idea about relative doubling time of various populations see link in course syllabus for Population Reference Bureau.

An examination of the World Population Data Sheets reveals that higher rates of natural increase are associated with high percentages of population under 15 years of age. Conversely, lower rates of natural increase are associated with higher percentages of population over 65 years of age. In general, lesser developed nation states have higher rates of natural increase, and developed nation states have lower rates of natural increase.

Period measures versus duration of exposure measures.

Difference between cross-sectional and longitudinal observations.

Unfortunately, the crude rate is not a very specific measure of what is happening in the actual distribution of demographic advance within the population. For instance, two countries that have the same crude death rate, but very different age distributions. Country A may have a younger population with 30 to 40 percent of its population under 15 years of age. Country B may have an older population with 20 percent of its population 65 years of age and over. Both countries may have the same crude death rate. To develop a more precise measure of the distribution of deaths in each country we need to construct age - specific death rates.

The principal specificity

Specific rates are developed to match the number of events that occur to a special subgroup of the population divided by the number of individuals at risk of experiencing an event in that population subgroup.

It is possible to extend the number of characteristics that define a special subgroup. Each additional characteristic adds increased detail about the composition of that population subgroup experiencing an event and the population at risk of suffering that event. For instance, we can generate age-specific rates; we can specify these rates further by sex. This would give us age- sex specific rates. We also could add marital status, thereby generating age- sex- marital specific rates. The greater the detail that we specify in delineating which group is at risk and which group suffers an event, the more accuracy is achieved in the calculation of rates.

With the use of this principle of specificity, let us examine the age-specific death rates for countries A and B. Age-specific rates are defined as follows: the number of events that occur to individuals of age x divided by the number of individuals in the population of age x, then multiplied by a constant.

ASDR = (d_x / P_x) * 1,000. Where x = age, d= number of deaths and P = number of population.

A comparison of the age-specific death rates in countries A and B reveals that: in country A the proportions of deaths which occurred in the first few years of life would be far higher than the proportion of the deaths which occurred in the first years of life in country B. This pattern is reversed looking at the age-specific death rates for the older ages in countries A and B.

Indirect standardization is a technique used to control for the influence of age structure when comparing age-specific rates between populations.

(_{known event}P_{sub-category i1}+ ..... +_{known event}P_{sub-category ith})* _{at risk for event}N_{total in category}=(_estN_{sub-category i1} + ......+ _estN_{sub-category ith})