Random assignment of participants to the experimental and control groups increases the likelihood that

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Random assignment of participants to experimental and control groups is a fundamental aspect of experimental design that helps ensure that the two groups are as similar as possible at the start of the experiment. This similarity allows researchers to more confidently attribute any differences in outcomes to the effects of the independent variable, rather than to preexisting differences between the groups.  It helps eliminate confounding variables related to individual differences among the research participants. Each group is likely to be similar because each participant has the same chance of being assigned to either group. For instance, using random assignment makes it unlikely that the groups will be divided so that one has all males and the other has all females; the groups are likely to be a mixture of males and females.

Random Assignment of Participants to the Experimental and Control Groups