The design selected must be appropriate to the study goals. Many research projects use variations of one of these approaches and in others a hybrid of two approaches might be suitable. Case series: describe a group of individuals with a disease. Cross-sectional study: describe exposure and/or disease status in a pop. Case-control study: compare exposure histories in people with disease (cases) and people without (controls. Cohort study : compare rates of new (incident) disease in pop. with different exposure histories or follow a pop. forward in time to look for incident diseases. Experimental study: compare outcomes in participants assigned to an intervention or control group. Qualitative study : seek to understand how individuals and communities perceive and make sense of the world and their experiences. Correlational (ecological) study : compare average levels of exposure and disease in several populations. The first critical decision is whether to conduct a primary, secondary, or tertiary study.