In experimental design, how are participants assigned to groups?

In experimental design, participants are assigned to groups randomly to ensure that each participant has an equal chance of being placed in any group. This random assignment helps to eliminate biases and control for confounding variables, thus allowing for a more reliable interpretation of the effect of the independent variable on the dependent variable. Random assignment increases the internal validity of the study, making it more likely that the results can be attributed to the treatment rather than other factors.

When participants are randomly assigned, the groups are more likely to be equivalent at the start of the experiment. This means that any differences observed at the end of the study can be more confidently attributed to the intervention or condition being tested rather than pre-existing differences among participants. This method stands in contrast to other assignment methods such as using prior academic performance, personal preferences, or teacher recommendations, which can introduce bias and lead to skewed outcomes based on those criteria.