What research method involves observing subjects without interference?

The correct choice is naturalistic observation, which significantly contributes to understanding how individuals behave in their natural environments without any manipulation or interference from the observer. This method allows researchers to collect data about behavior, interactions, and social dynamics as they naturally occur, leading to more ecologically valid findings.

In naturalistic observation, the focus is on capturing authentic behaviors and experiences as they unfold in real-world settings, such as homes, schools, or public places. This approach is particularly useful for studying phenomena that may not be ethically or practically examined through experimental methods, such as children's interactions in a playground or family dynamics.

Other methods listed, while valuable in research, do not involve observing subjects in a way that is entirely unintrusive. For example, experimental observation entails the researcher manipulating variables to establish cause-and-effect relationships, which means the environment is controlled and not purely naturalistic. Survey analysis relies on self-reported data, which can be subject to biases and may not accurately reflect actual behavior. Case studies focus on in-depth examination of individual subjects, often involving interaction and interviews, which again diverges from the non-intrusive observation characteristic of naturalistic observation.