If the amount of food is changed and fish growth is measured, what type of variable is the amount of food?

Correct answer: Independent variable

The direct answer is D, the independent variable. The independent variable is the factor that the experimenter deliberately changes. In this investigation, the amount of food is changed and fish growth is measured. Therefore food amount is the input or possible cause, while growth is the response. Step by step: choose different food amounts; keep fish type, number, water conditions, temperature, tank size and time as similar as possible; measure growth; compare growth between amounts; so food amount is independent. A is wrong because dependent variable means the measured response, here fish growth. B is wrong because “outcome variable” also refers to a result, not the changed food amount. C is wrong because a controlled variable is kept unchanged, whereas food quantity is intentionally varied. D is correct. A useful distinction is: independent = what I change; dependent = what I measure; controlled = what I keep the same. Memory cue: the cause is independent, the effect is dependent.