Which of the following distinguishes an observational study from a randomized experiment? (A)
In an observational study volunteers are always used, whereas in a randomized experiment a random sample
is always taken from the population.
(B) In an observational study a random sample is always taken from the population, whereas in a randomized
experiment volunteers are always used.
In
(C)
an observational study treatments are not randomly assigned, whereas in a randomized experiment
treatments are randomly assigned.
(D)
In an observational study a control group is never used, whereas in a randomized experiment a control group
is always used.
(E)
An observational study can be double-blind, whereas a randomized experiment can only be single-blind
because the experimenter determines who is randomly assigned to each treatment.