What is the primary purpose of randomization in experiments?

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Randomization aims to prevent bias by assigning participants to groups by chance, which helps make the groups similar at the start in both measured and unmeasured characteristics. When group assignment is random, any confounding factors—whether known or unknown—are likely to be spread roughly equally between groups. This balance means that differences in outcomes are more plausibly due to the intervention itself rather than preexisting differences. It also supports the use of statistical methods that assume random assignment to assess whether observed effects could occur by chance. It won’t maximize sample size or guarantee perfect measurement, and it may not achieve perfect balance in every study, but it moves the groups toward being comparable on average and reduces bias.