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| Case reports | Case series | Ecological studies | Cross sectional studies | Case control | Cohort | Intervention trials |
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Unit of study | (Single case) individual | (>1 cases) individual | Population | Individual | Individual | Individual | Individual |
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Description | Describes unusual characteristics of a case | A group of cases | Comparing populations in different places at the same time or, in a time series | Study of population at, “point-in-time,” | Study of two groups of subjects: (case; disease of interest and control; disease-free) | Study of two groups of subjects (exposure and non-exposure groups) | Study and examine two groups of subjects (intervention and control groups) |
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Direction | Present | Present | Present | Present | Reverse | Forward | Forward |
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Type of measurement | Reporting, description | Reporting, description | Correlation | Prevalence, association | Odds ratio | Prevalence, incidence, relative risk, attributable risk | Prevalence, incidence, relative risk, attributable risk |
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Advantages | Quick, having clinical importance, opportunities for physicians to exchange of thoughts | Quick, having clinical importance, opportunities for physicians to exchange of thoughts | Quick, inexpensive, group-level studies may also be the only way to study the effects of group-level constructs, for example, laws | Easy, inexpensive, useful for investigating fixed exposures such as blood group, most convenient in outbreaks of disease | Relatively inexpensive good for rare diseases Efficient in resources and time | Better for rare exposures, ability to determine causality relations | The strongest evidence for causality, control of unknown confounders, fulfils the basic assumption of statistical hypothesis tests |
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limitation | Inability to determine statistical relations and analysis | Inability to determine statistical relations and analysis | Ecological fallacy | Susceptible to selection bias and misclassification, difficult to establish a putative, “cause”, Not good for rare diseases or rare exposures | Susceptible to selection bias and misclassification bias, may be difficult to establish that, “cause,” preceded effect | Costly and time consuming, susceptible to selection bias. Relatively statistically inefficient unless disease is common. | Expensive, time consuming, sometimes ethically generalizability problem |
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