Consider the following three research questions:
What is the average mercury content in swordfish in the Atlantic Ocean?
Over the last 5 years, what is the average time to complete an undergraduate degree for Lancaster students?
Does a new drug reduce the number of deaths in patients with severe heart disease?
Each research question refers to a target population. In the first question, the target population is all swordfish in the Atlantic ocean, and each fish represents a case. Often times, it is too expensive to collect data for every case in a population. Instead, a sample is taken. A sample represents a subset of the cases and is often a small fraction of the population. For instance, 60 swordfish (or some other number) in the population might be selected, and this sample data may be used to provide an estimate of the population average and answer the research question.
For the second and third questions above, identify the target population and what represents an individual case.
Answer. (2) Notice that the question is only relevant to students who complete their degree; the average cannot be computed using a student who never finished her degree. Thus, only Lancaster undergraduate students who have graduated in the last five years represent cases in the population under consideration. Each such student would represent an individual case. (3) A person with severe heart disease represents a case. The population includes all people with severe heart disease.