In experimental design, random assignment primarily serves to...

In experimental design, random assignment primarily serves to...

• A. Distribute participants randomly to conditions.
• B. Select individuals with specific traits.
• C. Measure outcomes.
• D. Assign treatment based on clinician preference.

Answer: (B)

Random assignment creates groups that are equivalent at the start of the study, so differences in outcomes can be attributed to the treatment rather than preexisting differences. By giving each participant an equal chance of being placed in any condition, randomization balances both measured and unmeasured characteristics, reducing selection bias and strengthening internal validity. This is what allows researchers to make more confident causal inferences about the effect of the manipulated variable. Selecting individuals with specific traits would bias group composition, measuring outcomes happens after groups are formed, and assigning treatment by clinician preference would introduce systematic bias and undermine the study’s validity.

Random assignment creates groups that are equivalent at the start of the study, so differences in outcomes can be attributed to the treatment rather than preexisting differences. By giving each participant an equal chance of being placed in any condition, randomization balances both measured and unmeasured characteristics, reducing selection bias and strengthening internal validity. This is what allows researchers to make more confident causal inferences about the effect of the manipulated variable. Selecting individuals with specific traits would bias group composition, measuring outcomes happens after groups are formed, and assigning treatment by clinician preference would introduce systematic bias and undermine the study’s validity.