لخّصلي

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.

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. Primary studies can be either descriptive or analytical. 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. 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.

Case–control studies, also called “retrospective studies,” provide a relatively simple way to investigate the causal relationship between the predictor and outcome variables. A cohort study, on the other hand, follows a group of people exposed to some predictor variable with another group not exposed to the predictor to compare the outcome in these two groups. Besides determining causal relationship among variables, cohort studies can find the incidence of disease. Cohort studies are relatively expensive, time consuming, and not suitable to determining cause of rare diseases. Case–control studies, on the other hand, are best suited to determine the cause of rarer diseases, but have certain limitations as compared with cohort studies. Other hybrid studies, like the nested case–control and case–cohort studies, combine certain characteristics of cohort studies with other types of studies, are relatively less time consuming, and are cheaper than cohort studies in determining causal relationships, and some of them have fewer limitations than cohort studies.