A scientist studied a species of fish. She found that when a certain nutrient was added to the diet of the fish just after hatching, the fish gained an average of 3 kilograms in the first year. What additional information is needed to determine whether the nutrient is affecting the growth of the fish?

The questions have been asked in several important state and regional science exams in the United States. For example: TAKS-Texas Assessment of Knowledge and Skills, New York State Educational Department Regents exams, California Standards Test, etc.