In a plant growth experiment, which option is the independent variable: the color of light a plant is grown in, average increase in height for each light color, the species of the fern, the age and height of the plants, or the type of soil the plants are in?
The independent variable is the color of light the plant is grown in, because it is the factor you change on purpose to see how it affects plant growth. The average increase in height is the dependent variable (the measured outcome). The species, age/starting height, and soil type should be controlled variables if you want a fair test of light color.
What you are looking for in this question
The independent variable is the one the experimenter changes on purpose. Everything else should either be measured (dependent variable) or kept the same (controlled variables) so the test is fair.
Sorting each option into its role
- Color of light a plant is grown in: this is what you change between groups, so it is the independent variable.
- Average increase in height for each light color: this is the result you record, so it is the dependent variable.
- Species of the fern: should be kept the same (or carefully grouped) so it is a controlled variable.
- Age and height of the plants (starting conditions): should be matched as closely as possible, so they are controlled variables.
- Type of soil the plants are in: should be the same for all plants, so it is a controlled variable.
Quick check
If you ask, "What did I change?", the answer is "light color", so that is your independent variable.
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