Final answer:
In an ideal experiment, only one variable, the independent variable, is changed at a time, allowing the dependent variable to be measured against a control group to establish a cause-and-effect relationship.
Step-by-step explanation:
In a simple ideal experiment, only one variable is intentionally changed at a time. This ensures that any differences observed in the outcome, or dependent variable, can be directly attributed to the changes in the one manipulated, or independent variable. A control group is used as a benchmark to compare these outcomes against, where the control group does not experience the experimental manipulation. Random assignment to control or test groups is important to eliminate the influence of lurking variables, ensuring the validity of the cause-and-effect relationship established in the experiment.