An absolute number is the quantity or real size of a population's expansion. Subtracting the prior year's population from **the current year** yields an actual figure. For example, if the population was 100 last year and 150 this year, the absolute amount of increase is 50. This means that the population has expanded by **50 people**.

A percentage change is also called an "apparent" change because it shows up only when actual numbers are increasing or decreasing. For example, if the population was 100 last year and 150 this year, the apparent increase is 50%. Apparent changes can be misleading as well, since they don't take into account periods with increases and decreases in **the same year**. For example, if the population was 100 last year and 125 this year, then there was a real decrease of **5 persons**, even though the percentage change appears to be positive because there were more people last year than this year.

A relative change is shown as a ratio of one year's value divided by another. For example, if the population was 500 people in 1850 and 1000 people in 1900, the relative increase is 250%. Relative changes are useful for comparing amounts across years within the same country or region. They tell you how much the population increased or decreased as a proportion of its total size.

An abnormal change occurs whenever the current year's number is different from the previous year's number.

Absolute numbers I It is the magnitude of **population growth** over a year or decade. It is calculated by subtracting **the earlier and later populations**. (ii) Rate of **annual increase** It refers to the rate of population growth. It is expressed as a percentage each year. The higher the number, the faster the population is growing.

Relative numbers I It is the proportion of the population to the total number of people. For example, in 2017 the population of India was approximately 1.3 billion people and that of United States was approximately 323 million people. Thus, the population of India was about 13 times more than that of United States. Relative population density It is the number of people living in one area compared with another. In India, due to high population density, it is said that many Indians live in small areas.

Constant proportions I It is the ratio of the same two quantities at two different periods. For example, if the population of India increased from 100 million in 1950 to 200 million in 1970 and again increased to 300 million in 1990, then its constant proportion to the US population would be 2:3. That is, it would require twice as many people in India to keep the ratio between them and the Americans constant.

Decreasing proportions I It is the ratio of the same two quantities at two different periods.

Definition of absolute growth rate The increase in total growth of **two organs** or organisms is measured, and absolute growth is the comparison of total growth per unit time. For example, the total weight of two trees will increase at a constant rate if neither tree grows larger than the other. However not all increases are equal: some increases are due to **more tissue** while others are due to more mass. Mass is increased when a tree gets bigger, but size can also be increased by adding more branches or leaves. Only relative growth rates take into account these differences so that an increase in volume increases the rate of growth but not all growth is equivalent. Volume is increased when a tree gets taller, its diameter or girth increases.

Absolute growth curves show how much the total mass of an organism changes over time. They are usually shown as a function of time since an initial condition. For example, an absolute growth curve for two trees of different species and same age but of varying sizes would show that the smaller tree would have less mass at any given time because it was growing faster but eventually would reach the same size as the larger tree. After both trees were exactly the same age, the smaller tree would have more mass.

Absolute growth curves are useful for making comparisons between organisms of **different sizes**.

Rate of Population Growth It is computed by dividing the number of persons added to a population in a given year (natural increase + net in-migration) by the population size at the beginning of the year. The growth rate will be zero if births equal deaths and net migration is zero. If natural increase or net out-migration is greater than one, the population will grow.

Example: In 2001, the U.S. population increased by 3.5 million people. Of this number, 1.3 million were born and 2.0 million died. Calculate the rate of population growth.

First, find **the total number** of people in **the United States** on January 1, 2002: 3,849,000. Then divide the number of births minus the number of deaths by the number present on January 1, 2001: 1,331,000/3,849,000 = 0.034. The rate of **population growth** is therefore 0.034 (or 34 percent).

Note that when there are more births than deaths, as was the case in 2001, the population grows. When there are more deaths than births, as was the case in 1991, the population declines.

The rate of **population growth** can also be calculated by dividing the change in the population over time by **the original population**, expressed as a percentage.