When planning an experiment, which action ensures a fair test?

• A. Change multiple variables at once.
• B. Change only one variable at a time and keep all other conditions the same.
• C. Ignore control groups.
• D. Use as many measurement tools as possible.

Answer: (B)

To fairly test an idea, you change only one variable at a time while keeping everything else the same. This lets you see exactly how that one factor affects the outcome, because all other conditions are held constant and can’t influence the result. If you change more than one variable, you can’t tell which change caused any difference you observe.

Keeping the experiment’s other conditions constant also supports using a control or baseline to compare results, and using consistent measurement methods reduces extra sources of error. Changing many variables or skipping a baseline makes the test unfair, since the results could be due to any number of changes, not the one you want to test.