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

Non-experimental research involves the observation of variables without the manipulation that is central to experimental designs. In this type of research, researchers collect data based on natural observations or conditions rather than through controlled interventions. This means that researchers do not assign subjects or units randomly to different groups or conditions, making it impossible to infer cause-and-effect relationships with the same level of certainty as experimental research.

Quasi-experimental research shares similarities with experimental research, as it involves intervention or treatment; however, it often lacks random assignment to groups. This differentiates it from purely non-experimental approaches. In comparative research, researchers analyze and compare certain variables or groups, but this method may still involve elements of experimental or quasi-experimental designs depending on how the study is structured.