Final answer:
The manipulated element between experimental and control groups is the independent variable; it's the only factor that differs between the groups to measure its effect on the dependent variable.
Step-by-step explanation:
The one thing you change or manipulate between the experimental and control groups is the independent variable. The experimental group receives the manipulation of the independent variable, which is the treatment or factor being tested, while the control group does not. The purpose of this setup is to see if there are any changes in the dependent variable, which is what the researcher measures to see the effects of the independent variable. The control group may receive a placebo treatment, which serves to protect against bias by maintaining expectancy across both groups. Thus, any observed differences in the dependent variable can be attributed to the manipulated independent variable rather than chance.