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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A census, as mentioned by others, is an attempt at counting everyone resident in a country. Different countries use different definitions – in New Zealand, for example, they count everyone who is physically in the country in Census Day, whereas in the UK they’re only interested in people staying, or intending to stay, in the UK for 6 months or more.
Some people are counted more than once, such as children with parents who live apart. Others are not counted at all, for example people who refuse to respond, or who didn’t get out ask for a form, or who were on holiday at the time.
People called enumerators are hired to chase up non-responding households and encourage them to respond. Advertising plays a huge part. In many countries, it’s a legal requirement to fill in and return your census form.
To work out how many didn’t respond, a second, small-scale survey is done about a couple of months after Census Day. This is part of a method called “capture-recapture”, mentioned by u/marisbluesky – you match the results by name, address, sex, DOB (this is one of the main reasons for asking your name) and work out who you counted in one survey but not the other, who was counted in both, and from that work out who was missed in both. Then you extrapolate to the entire country’s population, which gives you your population estimate. Done well, the counted population in a census is about 95% of the estimated population.
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