The type of test that changes only one variable at a time in order to isolate the results is known as a _______.

• A. Descriptive study that observes without manipulation.
• B. Randomized trial
• C. Controlled experiment
• D. Case study

Answer: (C)

Isolating the effect of a single factor by changing only one variable while keeping all others constant is the hallmark of a controlled experiment. By manipulating the independent variable and holding everything else the same, you can see whether the variable you changed causes any observed effect. Including a baseline or control group that doesn’t receive the manipulation provides a comparison to show that the outcome is truly due to the variable you altered. This setup lets researchers draw cause-and-effect conclusions, which descriptive observations or in-depth case studies cannot do because they don’t control variables. A randomized trial fits within this framework as a type that uses random assignment to groups, but the key idea described—altering one factor at a time to isolate its effect—best matches the concept of a controlled experiment.