Which of the following is the primary difference between an experiment and an observational study?
A. Experiments are only conducted on human subjects; observational studies can be conducted on nonhuman subjects.
B. In an experiment, the researcher manipulates some variable to observe its effect on a response variable; in an observational study, he or she simply observes and records the observations.
C. Experiments must use randomized treatment and control groups; observational studies also use treatment and control groups, but they do not need to be randomized.
D. Experiments must be double-blind; observational studies do not need to be.
E. There is no substantive difference-they can both accomplish the same research goals.
