Question

In an attempt to prevent polio, Jonas Salk of the University of Pittsburgh developed a polio vaccine. In a test of its efficacy, a study was carried out in which nearly 2 million grade-school children were enrolled; half were given the vaccine, the other half received a placebo (in this case, an injection of salt water). Neither the children nor the doctors performing the diagnoses knew which children belong to which group. The incidence of polio was far lower in the group that received the Salk vaccine.

(a) (3 points) Is this statistical study an observational study or is it a randomized experiment? Explain
your answer.
(b) (2 points) Is it appropriate to conclude that the vaccine lowers the risk a child will develop polio? Why
or why not?

Answer 1

(a) This statistical study is a randomized experiment because the researcher had randomly assigned the grade-school children to an experimental group (the one which received the vaccine) or a control group (the one which received placebo). Such creation of homogenous groups helps a researcher to remove any sort of biases or judgments from the statistical study.

(b) Yes, it is appropriate to conclude that the vaccine lowers the risk of a child developing polio. We can say that because the sample sizes of the experimental group (receiving the vaccine) and the control group (receiving the placebo) were nearly 1 million. Since a sample is a representative part of a population and the sample sizes here are too large, we can say that the results derived from the sample are applicable on the population, as well.