Control Group vs. Controlled Variable Controlled Variable A controlled variable is one which is not allowed to change unpredictably during an experiment. Because they are ideally expected to remain the same, they are also called constant variables. Example An example of a constant variable could be the voltage from a power supply. If you are examining how electricity affects an experimental subject, you would keep the voltage constant, as otherwise the energy supplied would change as the voltage did. Control Group A control group is the experimental group tested without changing the variable. For example, to determine the effect of temperature on seed germination, one group of seeds may be heated to a certain temperature. The researcher will then compare the percent of seeds in this group that germinate and the time it takes them to germinate to another group of seeds (the control group) that have not been heated. All other variables, such as light and water, will remain the same for each group. Read more: http://www.answers.com/topic/experiment-3#ixzz1bELRgvYN Example In a trial of a drug or an experimental procedure, a group matched with the experimental group in all respects except the factor under investigation. A control group is an essential part of the scientific research method because it ensures that any changes observed in an experimental group are due solely to the drug or experimental procedure and not to any other factors.