When is randomization not applicable in study design?

Randomization plays a crucial role in study design, particularly in reducing bias and confounding variables, which enhances the validity of the results. It is a method often used in experimental designs where participants are assigned randomly to different groups to ensure that any differences observed can be attributed to the treatment or intervention rather than other factors.

When it comes to observational designs, such as case-control studies, randomization is not applicable. In these studies, researchers observe participants in their natural settings without manipulating any variables or assigning treatments to them randomly. This means that the groups are formed based on existing differences, such as whether individuals have a condition or not, rather than through random assignment. As a result, any confounding variables present cannot be controlled through randomization, and researchers must rely on statistical methods to adjust for these confounders.

Therefore, in the context of the question, the statement regarding when randomization is not applicable is accurately represented by observational designs like case-control studies. These designs inherently lack the random assignment feature that characterizes experimental studies and therefore cannot utilize randomization to eliminate biases.