system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do they determine the total human population of the world when so many nations are too underdeveloped or corrupt to be able to keep track of all their citizens effectively?
gpt
Census is the scientific term for taking account of a national asset such a population, number of households etc.

There is no one methodology for carrying out a population census. In most developed countries all citizens have social security number and births/deaths are recorded. This type of date gives a lot of insight into population statistics, such a growth pr. generation, fertility rate and more.

In countries where citizens do not have a social security number and where births and deaths are not tracked, a population census will have to do with proxy data.

Proxy data can be other data sources which can serve as a reliable indicator of population information. Such data will not be final and will involve uncertainties. 

One common proxy is questionnaires which are distributed not to all households but a representative selection. Questions in such a questionnaire will include, how many people live in a house hold, how many children are born in last year and how many have died.

Another typical source of proxy data is aerial and satellite photography. This data can give an insight into changes in settlements on a larger scale. If a city has grown/diminished can be used to derive amount of people living in a given area and other information.