What is the one factor that is different between the control group and the experimental group?
By definition, the control group is what the experimental group is measured against. For example, if one is doing an experiment to measure the effect of a drug on blood pressure, the experimental group would receive the drug and the control group would receive the placebo, or blank. The blood pressure of both groups would be measured after ingestion of either the drug or placebo and any differences in blood pressure we would attribute to the effect of the drug. Care must be taken, however, that the two groups are as identical as possible and that all procedures are carried out identically. Furthermore, to eliminate even more bias, both the researchers and the experimental subjects would not know whether they were receiving drug or placebo (a double-blind study).
