Establishing Causality
A true experiment is the only design that establishes causality: showing that changing one factor genuinely causes a change in another. The researcher achieves this through manipulation.
| Term | Role |
|---|---|
| Independent variable (IV) | The factor directly manipulated by the researcher |
| Dependent variable (DV) | The outcome measured to see the effect |
| Control group | The baseline group receiving no treatment |
| Confounding variable | An uncontrolled factor that might explain results |
Random assignment gives every participant an equal chance of entering the experimental group or control group. This randomization spreads hidden differences evenly, ruling out confounding variables.
Experimental Rigor
Several safeguards protect findings from bias and error. Operational definitions state precisely how abstract concepts are measured to ensure replicability.
A placebo controls for psychological belief in a treatment. A double-blind procedure hides group assignments from both participants and staff, protecting against experimenter bias where researcher expectations subtly alter data.
| Safeguard | What it Guards Against |
|---|---|
| Placebo | Psychological expectancy effects |
| Double-blind | Participant expectations and experimenter bias |
| Operational definition | Ambiguity and poor replicability |
Common pitfall: Assuming any two-group comparison is a true experiment. Without researcher-controlled random assignment, a study is merely a quasi-experiment and cannot establish strong causality.