Definición Population

In statistics, a population is the number of statistical units sharing at least one common property which is of interest in a statistical analysis.

An example population would be: everybody in Delaware who is at least 21 years of age. Since, for a survey, you cannot technically ask all of the people in Delaware, one usually selects a sample (sample survey), by which the opinions of 'people in Delaware over 21' are evaluated.

There are various selection methods you can use to select a sample. In small populations, for example the students at a high school, one could conduct a full population survey. In this case, one would speak of a census.

This rule applies for all populations: there must be a clear set of rules about who belongs to the group. For example when defining the group of  'everybody in Delaware over 21', one would have to determine if this includes only those who are registered with primary residence in Delaware or if the habitation of an apartment or a house in Delaware is crucial. Or, less likely, if it applies to everyone whose birthplace is Delaware.