AP Biology · Exam guide
AP Biology Experimental Design: Variables, Controls, and Predictions
A design question gives you a scenario and asks you to propose, critique, or improve an experiment. The answer is always built from the same parts, and a clear structure earns each point. This guide names the parts and shows a weak and a strong answer.
The parts of a design
| Part | What to write |
|---|---|
| Question or hypothesis | A testable statement that links one factor to an outcome. For example: if light intensity increases, then the rate of photosynthesis will increase. |
| Independent variable | The one factor you change on purpose. |
| Dependent variable | What you measure, with the units or method. |
| Controlled variables | Everything else that you keep the same, such as temperature and the amount of plant material. |
| Control group | A comparison group that does not get the treatment. |
| Replication | Repeated trials or enough samples, so chance has little effect. |
| Prediction | What you expect to see if the hypothesis is right, and what would show it is wrong. |
A weak and a strong answer
Prompt: Design an experiment to test whether the presence of salt in water slows the germination of radish seeds.
It has no comparison group, no measured outcome, and no controlled variables.
It names the variable, the control, the controlled factors, the measurement, the replication, and the prediction.
Critiquing a design
- Ask what else changed between groups besides the treatment.
- Ask whether the sample is large enough.
- Ask whether the measurement can distinguish the effect.
- Suggest a specific fix, not just a complaint.
Common mistakes
- Naming a dependent variable that is not measured.
- Changing two factors at once.
- Leaving out a control group or a control for each variable.
- Writing a prediction that cannot be tested.
Frequently asked questions
What is the difference between a control group and controlled variables?
A control group does not receive the treatment. Controlled variables are the factors kept the same across all groups.
Does a hypothesis have to be correct?
No. It has to be testable. The data decide whether it is supported.
How many trials do I need?
Several. The exam rewards saying that you repeat the experiment or use many samples.
Do I need numbers in my design?
Specific amounts, concentrations, and times make the design clear and earn credit.
Where can I find the official scoring rules?
Exam formats change a little from year to year, so check the current course and exam description on the College Board site before test day.
Keep practicing
Every unit in the AP Biology course ends with a quiz of multiple-choice and short free-response questions, with a writing space that scores against the four-point rubric and a model response to compare against. These free lessons also build the skills the free-response section tests:
See the AP Biology course Free lessons and lesson list
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Free-response and writing
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- AP Biology Graphing Question Explained: How to Construct a Graph and Interpret the Data
- AP Biology Short Free-Response Questions Explained: The Four Types and the 4-Point Rubric
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Multiple choice
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