When might a researcher opt for an observational design?

A researcher may choose an observational design specifically when studying pre-existing conditions is desired because this approach allows them to gather data about subjects in their natural environments without imposing any treatment or intervention. Observational designs are particularly useful for examining behavior or events as they naturally occur, which is essential when the aim is to understand phenomena that cannot ethically or feasibly be manipulated.

In cases where a condition already exists—such as behaviors affected by specific demographic factors or health conditions—it is inappropriate or impossible to assign participants to different conditions or treatments as would be typical in experimental designs. Instead, by observing these conditions, researchers can collect rich qualitative and quantitative data that provide insights into the dynamics at play without introducing additional variables that might cloud the results. This method also enhances the ecological validity of the findings, reflecting realistic behavior patterns and interactions.

The other options imply scenarios where manipulation or experimental control is possible or preferred, which does not align with the fundamental purpose of an observational design. Therefore, the choice of an observational method is strongly supported by the necessity to understand complex, pre-existing conditions in a non-intrusive manner.