A student decides to set up an experiment to determine the relationship between the growth rate of plants and the presence of detergent in the soil. He sets up 10 seed pots. In five of the seed pots, he mixes a precise amount of detergent with the soil. The other five seed pots have no detergent in the soil. The five seed pots with detergent are placed in the sun and the five seed pots with no detergent are placed in the shade. All 10 seed pots receive the same amount of water and the same number and type of seeds. He grows the plants for two months and charts the growth every two days. What is wrong with his experiment?
a) The student has too few pots.
b) The student has two independent variables.
c) The student has two dependent (resultant) variables.
d) The student has no experimental control on the soil.
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The experimenter has two independent variables: detergent amount and sunlight amount. The problem is that he is not testing all four possibilities. He has setup his experiment in a way that he cannot tell which of the two variables is relevant to his result. If the pots in the sun grow while those in the shade do not grow, is that because of the sun or because of the detergent?
He needs an experimental design that tests all possible combinations of independent variables.