Editor's Note: A guest comment on this article appears and the authors' response appears. With an annual incidence rate in the United States, glioma, which includes all primary brain tumors, is a rare but highly fatal disease 1 - 4 . Decades of research have provided examples. A case-control study is an observational study in which researchers analyzed two groups of people, cases and controls, to examine factors associated with particular diseases or outcomes. Below are some examples of case-control studies: Investigating the impact of daylight exposure on office health, case-control studies, due to their generally retrospective nature, can be used to correlate exposures and outcomes, but cannot establish causality. Epidemiology.