It is reasonable to use the entire population instead of a sample in the following situations:
1. A scientist wants to determine the average age of the trees in a national forest: Since the population is limited and easily accessible, it is feasible to collect data on all the trees in the forest.
4. A university Dean wants to estimate the number of students who work and study at the same time: Since the university Dean has access to student records, it may be possible to collect data on all students to accurately estimate the number of students who work and study simultaneously.