The variables trap
A free TEAS® scientific reasoning explainer with an original practice question
In an experiment, the independent variable is the one the researcher changes on purpose. The dependent variable is the result that gets measured. Controlled variables are kept the same in every group so they cannot explain the result.
The trap is picking the thing that is measured, or a thing that is held constant, because it appears in the question. Ask what differs between the groups by design: that is the independent variable.
A group that gets none of the treatment, such as 0 g of fertilizer, is the control group. It gives the other groups something to be compared against.
Try a sample question
A student tests whether the amount of fertilizer affects how tall bean plants grow. Three groups of bean plants get 0 g, 5 g or 10 g of fertilizer. All groups get the same water and light. Plant height is measured after 3 weeks. What is the independent variable?
- A.Plant height
- B.Amount of water
- C.Amount of fertilizer
- D.Amount of light
Show the answer
C. Amount of fertilizer
The amount of fertilizer is the independent variable. It is what the student changes on purpose between the groups. Plant height is the dependent variable, because it is measured. Water and light stay the same, so they are controlled variables.
Why the other answers tempt:
- A. Plant height is what gets measured, so it is the dependent variable.
- B. Every group gets the same water, so water is a controlled variable.
- D. Every group gets the same light, so light is a controlled variable.
The independent variable is the one you change. The dependent variable is the data you write down.
Checked against OpenStax Biology 2e, 1.1 The Science of Biology. Original question, not from a real test.
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