This can be calculated once we take a sample. In rare occasions the entire population is studied. This is a census . However, due to the huge cost involved only federal governments can undertake such a project. But the best we can do is get information from a sample.
Statistics helps in inferring about the population based upon the sample.
Suppose I am interested in the average height of students in this class. I sample 10 male students in my sample. We want the sample to be representative and not biased . Bias A sample is said to be biased when it systematically over or under represents some portion of the population. Agency A are interested in finding what the average income is in the Moscow-Pullman area.