What type of research is conducted when randomization of subjects to treatment and control groups is not possible?

When randomization of subjects to treatment and control groups is not possible, quasi-experimental research is the most appropriate approach. This type of research design allows for the comparison between groups that are not randomly assigned, often due to ethical, practical, or logistical constraints. By using existing groups, such as classes, teams, or communities, researchers can still draw conclusions about the effects of interventions or treatments by analyzing differences between these groups.

Quasi-experimental designs often employ pre-existing differences or match groups on certain characteristics to enhance the validity of the findings. This can help mitigate some of the threats to internal validity typically associated with non-random assignments, although causal inferences may be weaker compared to true experimental designs where randomization is present.

In contrast, experimental research requires randomization of subjects, meaning it cannot fit the scenario given. Action research involves a participatory approach aimed at solving practical problems and typically does not use control groups or randomization. Longitudinal studies focus on tracking the same subjects over a period to observe changes over time, rather than comparing different groups under varying conditions. Thus, given the conditions of the question, quasi-experimental research is indeed the correct classification.