In the AP Environmental Science exam, one of the three Free-Response Questions (FRQs) is almost always an experimental design prompt. These questions test your ability to apply environmental science concepts to a practical investigation scenario—something College Board loves because it blends critical thinking, data analysis, and scientific reasoning.
This RevisionDojo guide walks you through:
- What the experimental design FRQ looks like
- How to structure your response for maximum points
- Practice examples with breakdowns
- Common pitfalls and how to avoid them
1. Understanding the Experimental Design FRQ
An APES experimental design FRQ typically asks you to:
- Propose a hypothesis.
- Outline an experimental setup.
- Identify independent, dependent, and controlled variables.
- Explain methods for data collection.
- Suggest statistical analysis techniques.
- Interpret potential results.
Example prompt:
A scientist is testing the effect of different nitrogen fertilizer levels on algae growth in a pond ecosystem. Design a controlled experiment to determine the impact of nitrogen concentration on algae biomass.
2. How to Structure Your Answer
Step 1 – Restate the Problem & Hypothesis
- Write a clear, testable hypothesis in an “If…then…” format.
- Example: If nitrogen concentration increases, then algae biomass will increase due to higher nutrient availability.
Step 2 – Identify Variables
- Independent variable: What you change (nitrogen concentration).
- Dependent variable: What you measure (algae biomass).
- Controlled variables: Temperature, light, water volume, species of algae.
Step 3 – Experimental Setup
- Describe control and experimental groups.
- Specify replication (minimum 3 trials).
- Mention equipment and materials.
Step 4 – Data Collection Methods
- Explain how you’ll measure the dependent variable (e.g., chlorophyll concentration using spectrophotometry).
- Include time intervals and duration.
Step 5 – Data Analysis
- State the statistical method (e.g., ANOVA, t-test) or a graphical method (line graph, scatter plot).
- Justify why the method is appropriate.
Step 6 – Potential Errors & Improvements
- Mention possible sources of error (measurement inconsistencies, contamination).
- Suggest ways to reduce error (calibration, larger sample sizes).
3. Sample High-Scoring Response Breakdown
Prompt:
Design an experiment to determine the effect of pH on the growth rate of duckweed in a freshwater pond.
High-scoring approach:
- Hypothesis: If water pH decreases below neutral, duckweed growth rate will decrease due to enzyme inhibition.
- Variables: pH (independent), growth rate (dependent), temperature and light (controlled).
- Setup: Multiple tanks at pH 5, 6, 7, 8, each with equal duckweed mass, same light exposure, and temperature control.
- Data: Measure duckweed surface area coverage every 48 hours for 14 days.
- Analysis: Use mean growth rate and ANOVA to compare differences.
4. Practice Prompts for Self-Study
- Prompt 1: Test the effect of road salt runoff concentration on freshwater snail reproduction rates.
- Prompt 2: Investigate how different types of mulch affect soil moisture retention in urban gardens.
- Prompt 3: Examine how varying levels of dissolved oxygen impact fish survival rates in aquaculture tanks.
5. Common Mistakes to Avoid
- Forgetting replication: Experiments without replication lose points.
- Vague hypotheses: Must be measurable and testable.
- Not controlling variables: If too many factors change, results aren’t valid.
- Ignoring data analysis: Always say how you’ll process the data.
FAQ – Experimental Design FRQs in APES
Q: How much is the experimental design FRQ worth?
A: It’s typically worth about one-third of the FRQ section score.
Q: Should I draw diagrams?
A: Yes—if it clarifies your setup. Label everything clearly.
Q: Can I earn full points without statistical tests?
A: Possibly, but statistical analysis strengthens your answer and can earn more points.
Final Thoughts
The APES experimental design FRQ isn’t about memorizing facts—it’s about showing you can think like an environmental scientist. If you can identify variables, propose logical procedures, and justify your analysis methods, you’ll consistently score high.
For more FRQ breakdowns, scoring tips, and APES practice questions, check out RevisionDojo—your ultimate AP exam prep companion.
