Which of the following is an element of repeated measures design?

In a repeated measures design, the key feature is that the same participants experience multiple conditions or treatment levels throughout the study. This allows researchers to observe how the same individuals respond to different treatments, which enhances the sensitivity of the experiment by controlling for individual differences in responses. By using the same participants across various conditions, the design effectively minimizes variability that could arise from differences between subjects, thus leading to more reliable and valid results.

This approach is particularly beneficial in psychology studies where individual differences can significantly impact outcomes, as it allows for a smaller sample size while still providing robust data. The repeated exposure to different treatments can help in understanding changes in behavior or responses due to the varying conditions being tested.

In contrast, the other options reflect elements that do not align with the characteristics of a repeated measures design. For instance, the use of control groups is typical of independent measures designs where different groups of participants are compared. Changing groups during a study contradicts the concept of repeated measures, as it would involve different participants for each condition. Lastly, when each participant interacts with only one condition, it describes a completely different design structure known as an independent measures design, which is distinct from the repeated measures approach.