Which procedure reduces selection bias when assigning participants to conditions?

• A. Stratified random sampling
• B. Convenience sampling
• C. Quasi-experimental design
• D. Random assignment

Answer: (D)

The main idea is how to minimize biases that come from who ends up in which group. Random assignment does this by giving every participant an equal chance of being placed in any condition. When groups are formed randomly, individual differences—whether known or unknown—tend to cancel out across groups, so differences in outcomes are more likely due to the manipulated variable rather than pre-existing characteristics.

Stratified random sampling is about selecting a representative sample from the population, not about how you allocate participants to groups once they’re in the study. Convenience sampling relies on readily available participants, which can create biased groups. A quasi-experimental design lacks true random assignment, so pre-existing differences between groups can influence results. Random assignment specifically targets reducing those biases during the allocation to conditions.