The Design of Experiments
Observational studies can only show association. To prove standard causation, we must intervene. This is the art of the Experiment.
The Language of Experiments
Experimental Unit
The person, object, or well-defined item to which a treatment is applied.
Called a Subject if human.
Factor
A variable whose effect on the response variable is of interest.
LogicLens: Factor vs. Treatment
Students often confuse the variable with the outcome.
- Factor:The broad category (e.g., "Dosage").
- Level:Specific values (e.g., 0mg, 50mg, 100mg).
- Treatment:The specific combination applied (e.g., "50mg Dosage").
Control & Blinding
Control Group Baseline
A group that receives no treatment (or a standard treatment). It serves as a baseline to compare against.
The phenomenon where subjects show improvement simply because they *believe* they are receiving treatment.
Blinding (The Solution)
Design Frameworks
Completely Randomized Design
Experimental units are assigned to treatments purely by chance. No grouping. Simplest method, but high variability.
Randomized Block Design
Group (block) similar individuals together first (e.g., Men / Women), THEN randomize treatments within each block.
Matched-Pairs Design
A special case of blocking where the "block" size is exactly 2.
Type A: Two similar subjects (Twins).
Type B: One subject does both treatments (Before/After).
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