In an investigation different soil types and different amounts of water were used for every plant. What is the main problem?

Correct answer: Several variables were changed at the same time

When both soil type and water amount are changed, the cause of any difference in plant growth cannot be identified separately. In a fair test, only one independent variable is changed at a time while other factors are kept the same. Option B is incorrect because a dependent variable such as growth can still be measured; the problem is identifying what caused the change. Exam tip: Remember the rule ‘change one variable at a time’ for a fair test.