What key assumption is made in a within-subjects design?

In a within-subjects design, the key assumption is that each subject is exposed to multiple conditions or levels of the independent variable throughout the course of the study. This approach allows researchers to compare the responses of the same individuals across various conditions, controlling for individual differences that might otherwise confound the results. By having the same participants experience all levels of the independent variable, the design enhances the likelihood that any observed effects can be attributed to the experimental manipulation rather than to variability between different subjects. This setup is particularly advantageous in psychological research, where individual differences can significantly influence outcomes.

The other choices, while relevant to experimental design, do not capture the essence of within-subjects designs. Only allowing subjects to participate in one level of the independent variable contradicts the fundamental principle of this approach. While random assignment is beneficial for minimizing biases in between-subjects designs, it’s not a requirement for within-subjects designs since every subject acts as their own control. Finally, while within-subjects designs often involve multiple measurements, individual designs can vary and may at times involve single measurements per condition, making this choice inaccurate in the context of the core assumption.