Final answer:
An observational study observes pre-existing groups without random assignment, whereas longitudinal research follows the same participants over time.
Step-by-step explanation:
A research study in which no random assignment to levels of an independent variable is made, due to the presence of pre-existing groups, is known as an observational study. This is distinct from a longitudinal research study, which involves following the same group of participants over an extended period of time to observe changes or developments. Observational studies are necessary when it is neither ethical nor possible to manipulate variables for experimental research. Researchers rely on natural occurrences and measurements to draw relationships between variables. An example of this could be using surveys to assess behaviors or outcomes without influencing them directly.
While longitudinal research provides insights into how subjects change or develop over time, cross-sectional research compares different groups at a single point in time. Both these methods, along with natural observations and archival research, are key tools in the arsenal of non-experimental scientific investigation but serve different purposes based on the research question.