In experiments where individual differences are likely to be present and a small sample size is used, random assignment may not be sufficient to control for these differences.

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Random assignment is a method to minimize biased subject assignment to groups in an experiment, but it may not control for individual differences that exist within the sample. In cases where individual differences are likely to be present (such as in studies involving personality traits, intelligence, or prior experiences), the use of random assignment alone may not be sufficient to ensure that the groups are equivalent.

In Experiments Where Individual Differences Are Likely to Be Present