How do randomized controlled trials differ from quasi-experimental designs?

• A. A quasi-experimental design uses random assignment in some cases.
• B. A randomized controlled trial uses random assignment; quasi-experimental designs lack random assignment.
• C. Both designs rely on randomization to establish causality.
• D. A randomized controlled trial cannot assess causal effects.

Answer: (B)

Random assignment is the key factor that separates randomized controlled trials from quasi-experimental designs, and this difference directly affects how confidently we can claim causal effects. In a randomized controlled trial, participants are randomly assigned to receive the intervention or to a comparison condition. This randomization creates groups that are, on average, similar at baseline, so observed differences after the intervention are more plausibly caused by the treatment rather than preexisting differences.

Quasi-experimental designs do not use random assignment. They try to infer causality using nonrandom allocation or natural experiments, which means the groups being compared might differ in important ways before the intervention. Because of that, causal conclusions from quasi-experimental designs are more tentative and rely on additional assumptions or statistical adjustments to account for potential confounding.

So the statement that best captures the difference is that random assignment is present in randomized controlled trials, while quasi-experimental designs lack random assignment. The other descriptions either misstate the role of randomization or mischaracterize what randomized trials can do.