In a controlled experiment, a randomly selected set of children would be immunized while a control group would not be, and the difference between the outcome for the immunized group and the control group could be deemed a causal effect of immunization.
In acontrolled experiment, a randomly selected set of children would be immunized while a controlgroup would not be, and the difference between the outcome for the immunized group and thecontrol group could be deemed a causal effect of immunization.