The research objectives of a proposed study determine the type of study to be undertaken, either primary or secondary; primary studies are those that are actually performed by the investigators, while secondary studies summarize the results of different primary studies without actually performing the research.In cross-sectional studies, data collection is done at one point in time to determine prevalence of, and association between, variables but cannot find any causal relationship between the variables studied in terms of predictor (risk factor) and outcome.Descriptive studies, like case studies, case-series or surveillance studies, describe the occurrence of a disease (outcome) in the population without measuring the relationship between a risk factor (exposure) and a disease (outcome) and are often hypothesis-generating studies.Analytical studies, like cohort, case-control, and cross-sectional studies, measure relationships between exposures and outcomes and are performed to test hypotheses.Primary studies can be either descriptive or analytical.