Statistics and sampling
The complete group (that we are interested in).
If you want to find out about a complete population the best method is to ask everyone. A census of every household is a survey carried out every ten years by the Government. But this is very expensive and in 2011 the UK census cost the country $480 million. In 2010 the USA census cost $13 billion (WOW).
The population is all the people. The complete group of people.