Which of the following best describes a quasi-experiment?

A quasi-experiment is characterized by the absence of random assignment to groups, which distinguishes it from true experiments. In quasi-experimental designs, researchers may use pre-existing groups or may assign participants based on certain criteria rather than random selection. This approach is often adopted in settings where random assignment is not feasible for ethical or practical reasons, such as in educational or clinical contexts.

Quasi-experiments allow for the exploration of causal relationships where random assignment is not possible, but they may be subject to greater potential biases than true experiments due to the lack of randomization. For example, if a researcher is studying the effects of a new teaching method in two different classrooms, the choice of classrooms may influence the outcomes due to varying demographics, teaching styles, or other unrelated factors.

The other choices describe characteristics that do not align with the definition of a quasi-experiment. Random assignment and the presence of control groups relate more to true experimental designs, while observational studies involve observing subjects without manipulating variables, which is another distinct methodology.