A group of students designed an experiment to determine the effect of compost on the germination and growth of plants. The students set up experimental plots by mixing different ratios of soil and compost. They planted 20 pea and 20 melon seeds in each plot and watered each plot regularly. The students recorded the number of seeds that germinated, and as the plants grew, they recorded plant length, number and size of leaves, and general health observations.

The following observations were recorded.

• In all treatments, more pea seeds germinated than melon seeds.
• There was a week of rainy weather at three weeks.
• Melon plants developed a fungal growth at four weeks.
• Melon plants grew longer than pea plants, but many melon leaves showed signs of yellowing as the weeks passed.

The students concluded that pea plants grew better in compost than did melon plants.

Which of the following best addresses the validity of the conclusion made by the students?

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For a scientific experiment to be valid and lead to a conclusion such as causation, it needs to be controlled. There should only be one manipulated (independent) variable. To compare growth rates, we need to make sure all other variables are held constant. Since fungus appeared in the melon plots, we do not know whether the melon would have grown faster if the fungus was absent.

A large sample size may lead to more confidence of the results, but it does not mean the experiment is controlled.

Units of measurement are important, but does not change the general premise of the experiment.