Understanding Experimental Design and Variables

Key Concepts of Experimental Design

• Response Variable: Measures an outcome of a study.
• Explanatory Variable: Helps explain or predict changes in the response variable.
• Confounding: Effects of two variables on a response variable cannot be distinguished.

Study Types

• Observational Study: Collects data without influencing responses.
• Experiment: Imposes treatments to measure responses, ideal for showing cause and effect.

Experimental Vocabulary

• Treatment: Specific condition applied in an experiment.
• Experimental Unit: Object to which a treatment is randomly assigned.
• Subjects: Human experimental units.
• Factor: Variable manipulated in an experiment.
• Levels: Different values of a factor.
• Placebo: Treatment with no active ingredient; helps control expectations.
• Control Group: Provides a baseline for comparison; may receive placebo or no treatment.

Experimental Design Principles

• Comparison: Compare two or more treatments.
• Random Assignment: Assign experimental units randomly to treatments to create equivalent groups.
• Control: Keep other variables constant to avoid confounding.
• Replication: Use enough units to distinguish treatment effects from chance.

Experiment Types

• Completely Randomized Design: Units assigned to treatments purely by chance.
• Randomized Block Design: Assign units to treatments separately within blocks known to affect response.
• Matched Pairs Design: Pairs similar units and randomly assigns treatments within each pair or gives both treatments to each unit in succession.

Blinding in Experiments

• Double-Blind: Neither subjects nor those measuring responses know treatment assignments.
• Single-Blind: Either subjects or measurers do not know treatment assignments.

Statistical Significance

• Statistical Insignificance: A result may appear different due to small sample size or minor differences rather than a true effect.