Differences Between Populations

Abstract

Frequently an investigator wishes to compare or contrast two populations—sets of individuals or objects. This may be done on the basis of a sample from each of the two populations, as when average incomes in two groups, average driving skills of males and females, or average attendance rates in two school districts are compared. The polio vaccine trial compared the incidence rate of polio in the hypothetical population of children who might be inoculated with the vaccine and the rate in the hypothetical population of those who might not be inoculated; the two groups of children observed were considered as samples from these respective (hypothetical) populations. This example illustrates an experiment in which a group receiving an experimental treatment is compared with a “control” group. Ideally, the control group is similar to the experimental group in every way except that its members are not given the treatment.