Which design holds all other variables constant and uses random allocation to isolate cause and effect?

• A. True experiment
• B. Quasi-experiment
• C. Observational study
• D. Case study

Answer: (A)

The key idea is establishing causality through experimental control and randomization. This design manipulates one variable (the independent variable) and assigns participants to groups by random allocation, which helps ensure the groups are comparable at the start. Because the assignment is random, differences in the outcome can be attributed to the manipulation rather than preexisting differences or other factors. This combination—manipulation plus random assignment—maximizes internal validity and isolates the causal effect.

Quasi-experiments try to study effects without random assignment, so they can be influenced by selection biases and confounds. Observational studies simply observe existing conditions without manipulation, revealing associations but not proving causation. Case studies provide detailed information about a single case or small group but lack control over variables and randomization, limiting causal inference.