What type of research involves observations that cannot randomly assign units to conditions?

The correct choice focuses on non-experimental research, which is characterized by the absence of manipulation or controlled conditions that are typical in experimental research. In non-experimental research, researchers do not have the ability to randomly assign subjects or units to different conditions or groups. Instead, they typically observe and analyze variables as they naturally occur, without applying any treatment or intervention. This type of research is often used in fields where random assignment would be unethical or impractical, allowing researchers to gain insights into relationships and patterns based on existing conditions.

For example, studies that analyze existing data sets, conduct surveys, or observe behaviors in natural settings fall into this category. Because of its nature, non-experimental research can provide valuable descriptive or correlational insights but may lack the causal conclusions that experimental designs can offer.

In contrast, experimental research typically involves manipulating one or more independent variables to observe their effect on dependent variables, often using random assignment to establish control. Quasi-experimental research also involves the examination of cause-and-effect relationships but does not require random assignment, which differentiates it from pure experimental studies. Comparative research generally refers to studies that compare two or more groups or conditions but does not specify the method of assignment. Thus, non-experimental research is the most fitting