What method is used to minimize pre-existing differences among participants in an experiment?

Random assignment is the method used to minimize pre-existing differences among participants in an experiment. By randomly allocating participants to different groups, researchers ensure that each participant has an equal chance of being placed in any group, which helps eliminate biases that could influence the outcome of the study. This technique is vital for achieving comparability between the treatment groups, allowing for a clearer interpretation of the effects of the independent variable under investigation.

Using random assignment enhances the internal validity of the experiment, providing confidence that any observed differences in the outcome are likely due to the experimental manipulation rather than pre-existing characteristics of the participants. This method is particularly crucial in psychological and medical research, where individual differences can significantly impact results.

Other methods, such as systematic assignment, are less effective in addressing participant variability and could inadvertently lead to biased outcomes. Operational definitions clarify the variables used in the study but do not address participant differences, while counterbalancing is primarily used in experiments with repeated measures to control for order effects rather than to equalize groups before treatment.