There are two primary classifications of population data:

Primary population data collection sources: Data collected directly by a researcher or statistician or a government body via sources such as census, sample survey, etc. are called primary population data collection.

Secondary Population Data Collection Sources: Data obtained from existing sources such as journals, newspapers, magazines, annual research reports, etc. and not directly by a government or a research organization is called secondary population data collection.

The below-mentioned resources are the most used for human population data classification.

1. Census:

The simplest, yet not the most accurate or useful way to evaluate population is counting everyone. This is known as a census and is usually done by governments. In the past, religious organizations carried out censuses, but generally at a local or regional level. The Roman Empire carried out censuses to estimate the group of men of military age and for tax purposes, but these were limited since the Romans had to report with the government officials in their hometown to be counted. Poor people or those who could not travel were rarely counted.

A census is sometimes referred to as a complete enumeration. Each person is counted by personal interviews, surveys, or any other type of interview. There are no estimates.

Even a complete census has limits. In countries with very remote areas, it may be impossible for census takers to count everyone.

There are two ways of conducting census:

De facto method: When the census is conducted at an individual’s current residence, it is known as the de facto method. Generally, this census is carried out at night, and thus, it is also known as ‘one- night enumeration.’ It is conducted in urban regions in a country with high income.

De jure method: When the census is conducted at a person’s permanent residence, it is known as the de jure method. Compared to de facto, it is more practical and scientific and is also called ‘period enumeration.’ Nepal is one of the very few countries which follows this method.

Census has a list of the following attributes:

· Geographic segmentation attributes include current residence, permanent residence, place of birth, workplace information, etc.

· Personal and demographic details such as age, gender, marital status, literacy, language spoken at home, the number of people residing at home, etc.

· Information on an individual’s economic background, such as occupation, the current status of employment, primary source of income, etc.

2. Sample surveys:

An alternative to a full enumeration census is a sampling. You might be familiar with this as the method that market research companies and political analysts use to conduct their research. Statisticians use a mathematical formula to know the minimum number of people needed to constitute a population’s representative sample. For example, if the total population is 1,000 people, researchers could directly survey 150 of them. Then, they can take the data from the sample and extrapolate it to the entire population. If 10% of the sample people are left-handed, it can be assumed that 100 of a population of 1,000 are left-handed.

Sampling can yield more accurate results than full enumeration, but there are some caveats. All samples have a margin of error because of the possibility that samples differ from the total population.

This is expressed in terms of variation of error percentage, such as +/- 4%. The larger the sample size, the smaller the margin of error. Besides, samples must be chosen in the most unexpected way possible. This can be more difficult than it sounds.

Let’s say you want to survey a sample of all people in Johannesburg. A method used in the past was to select random names from the phone book. However, this eliminates the possibility of certain classes of people being selected for the sample: low-income people who do not have a telephone.

3. Administrative records:

The collection of population data from places that do not carry out censuses or historical periods where censuses were not common is achieved by gathering all available demographic information in administrative records. There could be partial censuses, data from the local population, or information collected by religious or civic groups. Examining birth and death records provides other clues.