What does an experimenter aim to control by random assignment?

Random assignment is a critical technique used in experimental research to ensure that participants are assigned to different groups (such as treatment or control groups) in a way that is not biased. This method aims to create equivalent groups at the outset of the experiment, allowing the experimenter to control for preexisting differences among participants.

Preexisting differences can include variabilities in demographics, psychology, health status, or any other aspect that might influence the outcome of the study. By randomly assigning participants, these differences are distributed evenly across the groups, reducing the likelihood that they will confound the results. This means that any observed effect can be more confidently attributed to the manipulation of the independent variable rather than differences that existed prior to the study.

In contrast, while participant expectations and environment can affect the outcome of an experiment, they are not the primary focus of what random assignment directly aims to control. Variability in dependent variables is also not directly managed by random assignment, as this technique predominantly addresses the composition of participant groups rather than variability in measures that result from the experimental manipulation.