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Three Principles of Experimental Design

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About This MicroSim

"Think of these three principles as the legs of a stool," Sylvia explains. "Remove any one, and your experiment falls over. Control, Randomization, Replication - they work together to make experiments trustworthy."

This interactive infographic presents the three fundamental principles of experimental design. Click each card to see a visual demonstration of how that principle works in practice, and toggle to see what goes wrong without each principle.

What You'll Learn

  • The definition and purpose of Control in experiments
  • Why Randomization balances both known and unknown variables
  • How Replication reduces the impact of individual variation
  • What happens when each principle is missing

How to Use

  1. Click any principle card to see a visual demonstration below
  2. Click "Show Problems Without" to see what can go wrong without each principle
  3. Hover over cards to preview selection
  4. Click a selected card again to deselect it

Understanding the Display

Principle Color Icon Purpose
Control Blue Lock Hold variables constant
Randomization Green Dice Use chance to assign treatments
Replication Auburn People Use enough experimental units

Key Concepts

Control

Control means keeping all variables constant except the one you are testing. This ensures that any observed differences between groups are due to the treatment, not other factors.

What to control: - Time and duration of treatment - Testing conditions and environment - Instructions given to participants - Materials and procedures

"If your treatment group studies in a quiet library and your control group studies in a noisy cafeteria," Sylvia notes, "how would you know if differences are due to your treatment or the environment?"

Randomization

Randomization uses a chance mechanism to decide which participants receive which treatment. This is powerful because it tends to balance ALL variables across groups - even ones you did not think of.

Why randomize: - Balances known confounding variables - Balances unknown confounding variables - Prevents selection bias - Creates comparable groups

Replication

Replication means using enough experimental units to see real patterns above random variation. With too few subjects, individual differences can overwhelm any treatment effect.

Benefits of replication: - Reduces impact of unusual individuals - Increases precision of estimates - Improves statistical power - Makes effects more convincing

Embedding This MicroSim

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Lesson Plan

Learning Objectives

By the end of this activity, students will be able to:

  1. Define and explain Control, Randomization, and Replication
  2. Describe the purpose of each principle in experimental design
  3. Predict what goes wrong when each principle is violated
  4. Apply all three principles when designing an experiment

Target Audience

  • AP Statistics students (high school)
  • Research methods courses
  • Science fair participants

Prerequisites

  • Basic understanding of experiments vs. observational studies
  • Concept of treatment and control groups
  • Awareness of confounding variables

Classroom Activities

Activity 1: Principle Matching (10 minutes)

Display these experimental flaws and have students identify which principle was violated:

  1. "We let students choose whether to use the new study method or the old one."
  2. "We tested our drug on just 3 patients."
  3. "Treatment group practiced in the morning; control group practiced at night."

Activity 2: Design Check (15 minutes)

Present an experiment design and have students verify all three principles:

A researcher wants to test whether background music affects reading comprehension.

  • What should be controlled?
  • How should randomization be done?
  • How many participants are needed?

Activity 3: Fix the Experiment (15 minutes)

Provide flawed experiment descriptions and have students redesign them to follow all three principles:

  1. "Students who want extra credit volunteer for the treatment group."
  2. "We tested the fertilizer on one plant and compared it to one untreated plant."
  3. "The new teaching method was used in a brand new classroom."

Common Mistakes to Address

  1. Confusing randomization with random sampling: Random assignment is not the same as random sampling
  2. Underestimating needed sample size: "A few subjects should be enough"
  3. Forgetting subtle controls: Time of day, room temperature, researcher behavior
  4. Thinking randomization guarantees identical groups: It balances groups on average

Assessment Questions

  1. A researcher conducts an experiment with just 5 participants. Which principle is most likely being violated?

  2. In a drug trial, doctors assign healthier patients to the treatment group. Which principle is violated and why?

  3. Explain why randomization is sometimes called "the great equalizer" in experimental design.

  4. Design an experiment to test whether classical music improves plant growth. Show how you would apply all three principles.

References