Which method ensures that participants have an equal chance of being assigned to any group in a study?

The method that ensures participants have an equal chance of being assigned to any group in a study is random assignment. This technique is crucial in experimental research because it helps to eliminate selection biases by distributing participants randomly across different groups. As a result, it increases the likelihood that groups will be equivalent at the start of the experiment, allowing researchers to make more valid causal inferences about the effects of the independent variable on the dependent variable.

Random assignment operates under the principle of chance, making it more likely that individual characteristics, such as age, gender, or other factors, will be equally represented across groups. This balance among groups plays a pivotal role in controlling for confounding variables that could distort the results of the study.

Other methods listed, such as systematic sampling and stratified sampling, do not focus on group assignment within the experimental design but rather on how participants are selected for inclusion in studies. Non-random assignment lacks the rigor needed to ensure groups are equivalent, which can lead to biased results and challenge the validity of the study's conclusions.