In a 'controlled' experiment, how many variables should you change at a time?

• A. Several variables
• B. No variables
• C. One variable
• D. All variables

Answer: (C)

When you want to know if a factor affects an outcome, you isolate that factor by changing only one thing at a time. In a controlled experiment, you intentionally alter a single variable (the independent variable) and keep all other factors the same (the controlled variables). This way, any difference in the result can be attributed to that one variable, because nothing else varied to cause a change.

Changing more than one variable at once introduces confounding factors, making it impossible to tell which variable is responsible for any observed effect. If you don’t change anything, there’s no test to learn from, and if you change everything, you lose the ability to identify cause and effect. So the best approach is to change one variable at a time.