What defines randomized controlled trials (RCTs)?

Randomized controlled trials (RCTs) are defined primarily by the random assignment of subjects to either treatment or control groups. This method is essential because it helps to eliminate selection bias and ensures that the groups are comparable at the outset of the study. This randomization process allows researchers to make causal inferences about the effects of an intervention or treatment, as it helps to isolate the variable of interest while controlling for confounding factors.

By assigning participants randomly, researchers can be more confident that any observed effects are due to the treatment being tested rather than other extraneous variables. This characteristic sets RCTs apart from other study designs, such as observational studies, where participants are not randomly assigned, and the researcher typically has less control over potential confounding factors.

The other options, while related to different aspects of research methodology, do not capture the defining feature of RCTs. Specifically, random selection for observational studies does not involve treatment assignments, qualitative focus lacks the quantitative rigor typical in RCTs, and categorizing RCTs as non-experimental contradicts their classification as a cornerstone of experimental research design.