How is world population calculated? And how accurate is the number? Is it possible that the number can be entirely wrong?
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As others have said, a census counts the number of residents in a country. However, there are a few issues:
1) Who counts as a resident?
New Zealand counts everyone in the country on Census Day, regardless of how long they are staying for. The UK counts everyone who has stayed, or intends to stay, for at least 6 months.
2) What about non-response?
Usually there are big advertising campaigns so that everyone is aware that there is a census, and why it’s important. There are people called enumerators who visit non-responding households to encourage them to return their form.
The number of responses is the *counted population*. However, some people don’t respond. They may refuse, they may forget, they may be on holiday. So a second small-scale survey is done, a few months later. This is part of the “capture-recapture” method mentioned by u/marisbluesky which is also used for animal population estimates. Identifying information (name, DoB, sex, address) is used to work out who responded in one survey and not the other, who responded in both, and hence estimate how many were missed in the census. This is used to produce the *population estimate* for the whole country.
If the census is done well, the counted population will be at least 95% of the estimated population.
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