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How to Design a Controlled Experiment
Grades 6-12 · 45 minutes
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A controlled experiment is a fair test that helps you find out whether one factor causes a change in another factor. It matters because school science projects should produce evidence, not just observations or guesses. By changing only one variable and keeping the rest the same, you can make a stronger claim about what caused your results. A clear experimental design also makes your project easier for others to understand and repeat.
In a plant-growth experiment, you might compare a control group under white light with an experimental group under blue light. The independent variable is the light color, the dependent variable is plant growth, and controlled variables include plant type, soil, water, pot size, distance from the lamp, and time exposed to light. Replication means using several plants in each group, not just one, so a single unusual plant does not decide the outcome. Careful measurements, a data table, and a graph help turn the experiment into a reliable conclusion.
Key Facts
- A controlled experiment changes one independent variable and measures one dependent variable.
- Control group: the standard comparison group that does not receive the tested change.
- Experimental group: the group that receives the changed condition being tested.
- Plant example: independent variable = light color, dependent variable = plant height or growth rate.
- Growth rate = change in height / change in time.
- Replication improves reliability because repeated trials reduce the effect of random variation.
Vocabulary
- Independent variable
- The factor the experimenter deliberately changes to test its effect.
- Dependent variable
- The factor that is measured to see how it responds to the independent variable.
- Controlled variable
- A factor kept the same in all groups so the test stays fair.
- Control group
- The group used as the normal standard for comparison.
- Replication
- The use of multiple trials or multiple samples in each group to make results more dependable.
Common Mistakes to Avoid
- Changing more than one variable at a time: this makes it impossible to know which change caused the result.
- Using only one plant in each group: one sample can be unusual, so each group should include several plants.
- Forgetting to define how growth will be measured: vague results like looks healthier are weaker than measurements such as height in centimeters.
- Treating the control group as optional: without a comparison group, you cannot tell whether the experimental condition made a real difference.
Practice Questions
- 1 A student grows 5 bean plants under white light and 5 bean plants under blue light for 14 days. The average height increases from 4 cm to 16 cm under white light and from 4 cm to 20 cm under blue light. What is the growth rate for each group in cm/day?
- 2 You test whether fertilizer affects radish growth. Group A gets no fertilizer and Group B gets 10 mL of fertilizer solution each week. Both groups have 8 plants. Identify the independent variable, dependent variable, control group, and one controlled variable.
- 3 A student changes light color, water amount, and soil type all at once in a plant experiment. Explain why this design cannot show which factor caused any difference in growth, and describe how to redesign it as a controlled experiment.