When Random Assignment Fails
Not every question can be studied with a true experiment. Researchers cannot ethically or practically assign people to smoke, experience trauma, or be born male or female.
Quasi-experimental designs compare groups that differ naturally, like a pre-existing condition, WITHOUT random assignment. A natural experiment compares regions before and after an uncontrolled policy change. They are informative, but weaker at ruling out confounds than true experiments.
Non-experimental designs involve zero variable manipulation.
| Design | Manipulation? | Random Assignment? | Can Establish Causation? |
|---|---|---|---|
| True Experiment | Yes | Yes | Yes |
| Quasi-Experiment | Sometimes | No | Weakly, at best |
| Correlational | No | No | No |
| Case Study | No | No | No |
Non-Experimental Designs
Correlational studies measure the relationship between two variables as they naturally occur, such as hours of sleep and reported mood.
Critical Pitfall: Treating a strong correlation as proof of causation. A correlation could mean variable A causes B, B causes A, or a third variable (like chronic stress) causes both.
Case studies examine a single individual or small group in great depth, perfect for rare phenomena like an unusual brain injury. Archival research analyzes existing records or data collected for another purpose entirely.