What is the role of random assignment in experiments?

• A. To determine the dependent variable
• B. To ensure equivalence of groups at the start of the experiment
• C. To guarantee the results are externally valid
• D. To ensure the sample represents the population

Answer: (B)

Random assignment places participants into experimental groups by chance. This helps ensure the groups are comparable on average at the start, so any differences in the outcome can be attributed to the manipulation rather than preexisting differences. In other words, it controls for confounding variables you can’t measure, boosting internal validity.

The dependent variable is the outcome you measure, not something you assign. External validity isn’t guaranteed by random assignment; it’s about whether findings generalize beyond the study, which depends more on the sample, setting, and procedures. Similarly, representing the population comes from how you select participants (sampling), not how you allocate them to groups.