Random assignment aims to minimize bias by ensuring that

• A. Each participant has an equal chance of assignment to any group
• B. Participants choose their group
• C. Researchers assign groups based on preference
• D. Groups are fixed before enrollment

Answer: (A)

Random assignment is used to create comparable groups by chance, which minimizes bias in an experiment. When every participant has an equal chance of ending up in any group, the groups are more likely to be similar in both known and unknown characteristics. This balance makes it more likely that observed differences after the study are due to the intervention itself rather than preexisting differences between participants.

If participants choose their group, self-selection bias can distort comparisons because people who opt for a particular group may differ in important ways. If researchers assign groups based on preference, that introduces systematic bias because the assignment is not random. If groups are fixed before enrollment, there’s no randomization to balance characteristics, allowing preexisting differences to drive outcomes.