An experiment or clinical trial in which two groups are used for comparison
In a controlled exposure study, one group of participants is
exposed to a substance (e.g. a pollutant) while those in the
"control" group are not. The results are then
compared to determine the health effects of the substance being studied.
In a placebo controlled study (often called simply controlled
study), people in the "treatment" group are given the drug or treatment being
tested while the control group receives a placebo or a different treatment.