Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a census (a process of collecting, analysing, compiling, and publishing data regarding a population).A population pyramid or "age-sex pyramid" is a graphical illustration of the distribution of a population (typically that of a country or region of the world) by age groups and sex; it typically takes the shape of a pyramid when the population is growing.Males are usually shown on the left and females on the right, and they may be measured in absolute numbers or as a percentage of the total population. The pyramid can be used to visualize the age of a particular population.It is also used in ecology to determine the overall age distribution of a population; an indication of the reproductive capabilities and likelihood of the continuation of a species. There are three main trends in populations that affect the shape of a population pyramid. The first is when there are both high fertility and high mortality rates among younger members. This type of population, known as “expansive,” creates a sharp triangle shape in the graph. Expansive pyramids mean that the population does not increase much in total number and has many young people. The second trend, known as “constrictive,” is when there is a lower mortality rate with the fertility rate remaining constant. These population pyramids are wider in the middle of the graph as the population has high numbers of middle aged and elderly people, but fewer young people. The third trend is “stationary” which is a population with low mortality and low fertility rates. These graphs have a square or “pillar” shape rather than a pyramid one. These population pyramids represent a stable population that will not change significantly barring any sudden changes to fertility or mortality rates.