Number of elements or individuals in the population that is the subject of a statistical study.
Example
At a school, there are 220 students: 120 girls and 100 boys.
For a survey, we decided to question one-quarter of the girls and one-quarter of the boys.
Therefore, we surveyed 30 girls and 25 boys, chosen at random.
The population size is 220 and the sample size is 55.