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

The method used to minimize pre-existing differences among participants in an experiment is random assignment. Random assignment ensures that each participant has an equal chance of being placed in any experimental group, which helps to distribute individual differences (such as age, gender, socioeconomic status, etc.) evenly across all groups. This process aids in creating comparable groups, reducing the potential for bias and enhancing the internal validity of the experiment by ensuring that the observed effects can more confidently be attributed to the treatment or manipulation being studied, rather than to differences among participants.

In contrast, systematic assignment would involve allocating participants into groups based on some predetermined criteria, which could inadvertently perpetuate existing differences. An operational definition refers to the specific way in which a concept or variable is measured or defined in a study, but it does not address participant selection. Counterbalancing is primarily used to control for order effects in within-subject designs by varying the order of treatment conditions, rather than minimizing pre-existing differences.