A biologist wants to determine the effect of a new fertilizer on tomato plants. What is the best design for this study?

First, the biologist must define the variables:

Dependent - this is what will be measured. So the biologist may want to measure the weight and height of the tomatoes, or the leaf area or the time it will produce the fruit.

Independent - this is what will differ in the experiment and what the biologist thinks will affect the dependent variable. In this case, the biologist will use new fertilizer on plant A, old fertilizer on plant B, and no fertilizer on plant C.

Controlled - these variables must be held constant because the biologist doesn’t want other factors to affect the growth of the tomato plant, thus, destroying the purpose of the experiment, tomato plant A, B, and C will have equal amounts of soil, water, and carbon dioxide, and exposure to sunlight.

After his experimentation, he will conclude which of the plant has better weight, growth, leaf, and fruit. If Plant A is the best tomato plant of them all, then he can say that the new fertilizer is more effective.