A student investigated the effect of the color of light on the growth of plants. Red, yellow, and green lights were used in the experiment. If another student were to conduct the same experiment, which would be the most important procedure to follow to yield the same results?

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.