Comment

It is important to distinguish between a population parameter and a sample statistic. A parameter is a numerical summary of a population; because populations have so many members, these can never be known except if a census is taken. For example, it would be impossible to know the average summer earnings of all university students. This would require us to identify, find, and question many millions of students. But it is feasible to select a sample of 100 students (using proper randomization, replication, and blocking techniques) and then the average earnings of these 100 students could be computed. Any numerical measure computed from a subset of the population (typically a sample) is a statistic.