Human studies on aging, health, and functioning are commonly categorized into observational studies and intervention studies. Observational studies aim to investigate the incidence, prevalence, and risk factors for specific outcomes and their interaction with factors like the living environment or social context, without attempting to influence these phenomena. Observational studies are often criticized for not being able to produce information on causality. Instead, observational studies will create knowledge about how common a condition is, what coincides with it, are the risks realized similarly in different environments and among different groups of people, and do some factors exacerbate or reduce the consequences of risk factors. These insights are needed when planning intervention studies. Intervention studies seek to modify the phenomenon of interest to confirm the results observed in observational studies and confirm causality. The research aim always underlies the decisions about the research design, which will be selected based on its ability to best answer the research questions posed.