Introduction: Why Experimental Design Matters
AP Statistics is not just about crunching numbers — it’s about designing studies that produce trustworthy data. Experimental design questions appear every year on the exam, often in FRQs.
If you know how to:
- Identify good vs. bad study designs
- Explain randomization, replication, and control
- Write clear responses in context
…you’ll earn easy points, even without heavy math.
This guide breaks down experimental design step by step using RevisionDojo’s proven strategies.
What Is Experimental Design?
Experimental design refers to how researchers set up a study to collect valid, unbiased data.
The goal: establish cause-and-effect relationships.
Key difference:
- Observational study: Watch without interfering → can show correlation.
- Experiment: Manipulate a factor → can show causation.
Principles of Experimental Design
There are three core principles every AP Stats student must know:
- Control
- Keep other variables constant to isolate the effect of treatment.
- Example: If testing a new fertilizer on plants, keep sunlight, water, and soil the same.
- Randomization
- Assign subjects to treatments randomly to reduce bias.
- Ensures groups are similar at the start.
- Replication
- Use enough subjects (or repeat experiment) to reduce variability.
- More subjects = stronger conclusions.
RevisionDojo mnemonic: “CRR = Control, Randomize, Replicate.”
Types of Experimental Designs
1. Completely Randomized Design (CRD)
- Subjects randomly assigned to treatments.
- Example: 100 volunteers randomly assigned to Drug A or Drug B.
2. Randomized Block Design (RBD)
- Subjects grouped into blocks based on a variable, then randomized within blocks.
- Example: Block by gender, then randomly assign males/females to treatments.
3. Matched Pairs Design
- Each subject receives both treatments (order randomized), or matched with another subject with similar traits.
- Example: Each student tries studying with and without background music.
RevisionDojo tip: Matched pairs are special cases of block designs.
Key Vocabulary for AP Stats
- Explanatory variable: The factor being manipulated (independent variable).
- Response variable: Outcome measured (dependent variable).
- Confounding variable: A factor that could affect results but isn’t controlled.
- Placebo effect: Subjects respond to “fake” treatment due to expectations.
- Blinding: Subjects or researchers don’t know treatment assignment.
- Double-blind: Neither subjects nor evaluators know assignments.
These terms appear constantly in FRQs.
Example FRQ Walkthrough
Scenario: A researcher wants to test if a new teaching method improves AP Stats scores.
- Identify variables:
- Explanatory = teaching method (traditional vs. new).
- Response = AP Stats exam score.
- Choose design: Randomized block by prior GPA.
- Assign treatments: Within each GPA block, randomly assign half to traditional and half to new method.
- Control: Same teacher, classroom, and test format.
- Replication: Use at least 50 students per block.
- Interpretation: If new method group scores higher, can conclude teaching method caused improvement.
RevisionDojo provides sample FRQ answers with scoring guidelines so you can practice writing like an AP grader expects.
Common Mistakes Students Make
- Confusing observational studies with experiments.
- Forgetting random assignment (essential for cause-and-effect).
- Ignoring placebo or blinding in medical studies.
- Writing vague answers like “test one group” instead of specifying randomization.
- Not linking back to cause-and-effect.
RevisionDojo’s Experimental Design Strategy
To answer design questions quickly, use the C-R-R-B checklist:
- C = Control: Keep other variables the same.
- R = Randomize: Assign treatments randomly.
- R = Replicate: Use enough subjects.
- B = Block (if needed): Group by relevant characteristics.
This structure matches exactly what graders want.
Experimental Design vs Sampling Methods
Don’t confuse experimental design with sampling:
- Sampling methods (Unit 3): How you select a sample (SRS, stratified, cluster, systematic).
- Experimental design (Unit 4): How you assign treatments once you have subjects.
Both are tested, but experimental design is all about cause-and-effect.
Calculator Tips
Even though experimental design is mostly conceptual, calculators help with:
- Random number generators → assign subjects randomly.
- Simulations → model treatment outcomes.
RevisionDojo guides include TI-84 and Desmos tutorials for these tools.
How Experimental Design Connects to Later Units
- Inference: Random assignment justifies using inference tests.
- Confidence intervals: Replication reduces variability, making intervals narrower.
- Hypothesis testing: Design ensures p-values are valid.
Experimental design is the foundation of AP Stats inference.
Exam-Day Checklist for Experimental Design Questions
- Identify explanatory + response variables.
- State treatment groups clearly.
- Mention control, randomization, and replication.
- Use block or matched pairs if relevant.
- Write in full sentences with context.
Frequently Asked Questions (FAQs)
Q: What’s the difference between random sampling and random assignment?
A: Random sampling selects subjects from a population. Random assignment allocates subjects to treatments in an experiment.
Q: Why is blinding important?
A: Prevents bias from subjects or researchers affecting results.
Q: Do I always need a placebo?
A: No, only in studies where expectations could influence results (like medicine).
Q: How do I know when to use blocks?
A: If there’s a variable that strongly affects response (e.g., gender, GPA), block on it.
Q: Will experimental design always appear on the exam?
A: Almost certainly — usually as a multi-part FRQ.
Final Thoughts
Experimental design is a high-yield topic on the AP Statistics exam.
- Master the principles: control, randomization, replication.
- Know designs: completely randomized, block, matched pairs.
- Write clear, contextual answers on FRQs.
With RevisionDojo’s guides, FRQ practice, and C-R-R-B strategy, you’ll be ready to tackle any experimental design problem and secure those crucial points toward a 5 on AP Statistics.
