I.  Demography is the study of  populations.  Topics include the age structure of populations, mortality (death) rate and fertility
        (birth) rates, and the factors which influence these factors.

Populations can be studied at a number of different scales, including:

Species; which includes all individuals on Earth which can interbrred and have fertile offspring

Local populations; which are a group of individuals within the same species which live together in the same territiory but do not commonly interbreed

Demes; which are groups in which interbreeding occurs within a local population

Cohorts; which are groups of individuals of similar ages

At any of these scales, a number of  variables can be calculated, including:

Net Reproductive Rate; which is the average number of  offspring born per adult in a lifetime

Mean Life Expectancy; which is the average number of years left to be lived from a given age

Mean Generation Time; which is the average number of years between an adult's birth and the birth of that individual's middle child

Some population statistics are best represented graphically.  One of the easiest to interpret are Age Structure diagrams.

To make this type of graph, you must:
 

(1) Count the total number of individuals in a population

(2) Divide the population up into an equal number of age classes (cohorts).  For human populations, these are
        often 5-year periods.

(3) Within each cohort, count the number of females and males.

(4)  For each cohort, divide the number of males and females into the total population size.

(5)  Graph these numbers, with the vertical axis representing cohort age, and the horizontal axis representing the
        proportion of the population held within each cohort.  It is common practice to put the percentage of males
        on the left side of the graph, and the percentage of females on the right.

Much about the potential for future population growth, and of the history of  of a population can be determined from the shape of Age Structure diagrams.

Population growth rates:

(1) Rapidly growing populations will have a strongly triangular shape.  This means that each younger cohort is
        larger than the one preceeding it.  Thus, the largest number of individuals in a population will be infants.

(2) Slowly growing populations will have approximately equal sizes among the younger cohorts, but will have a
        strongly pyramidal shape in the older cohorts.  This kind of diagram is created when a rapidly growing
        population's birth rate falls to replacement levels (on average 1 child born per adult in a lifetime)

(3) Stable populations will have all but the oldest cohorts having approximately equal sizes.
Disasters in a population's history can be identified by looking for cohorts that are much smaller than the cohorts on either side.  The size of these cohorts is smaller as individuals were killed during the disaster.

Patterns of immegration can also be identified by looking for:

(1) some cohorts which are much larger than their neighbors

(2) the shape of the male side differing greatly from the female diagram.  This is happens in immegration, as both
        sexes usually migrate at different rates.