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Basic concepts in epidemiology

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This describes the proportion of persons in a population that suffer from a specific disease at a defined point in time or during a period of time.

Prevalence = Number of diseased persons

Total number of persons in the population

The prevalence can be stated both as a proportion and as a percentage (Carranza et al. 2006).
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(= risk, cumulative incidence)

Average percentage of persons without disease in a population with risk factors, who develop disease in a given period of time. Incidence can be regarded as the risk or probability of developing disease.

Incidence = Number of new disease cases

Number of persons in the population with risk factors

(Carranza et al. 2006)

Brief overview of epidemiological study types:

Depending on the scientific question, epidemiological studies are designed as descriptive, analytic or experimental/evaluative epidemiological longitudinal or cross-sectional studies (Micheelis et al. 2006).


The following are the characteristics of the most frequent study types:

Cross-sectional study

Cross-sectional studies investigate every study participant only at a single point in time (i.e., no recording of data from the past or future, no follow-up investigations). While cross-sectional studies of descriptive questions (e.g., prevalence of a disease) can often provide reliable information, their importance is limited in the case of analytical questions. Because exposure (e.g., the presence of a risk factor) and disease are recorded simultaneously in a cross-sectional study, it is unclear whether an association between both variables was actually caused by a causal relationship between exposure and disease; a cross-sectional study cannot demonstrate that the disease did not lead to an increased occurrence of the exposure or that a third variable has a causal influence on both variables (Deutsches Netzwerk Evidenzbasierte Medizin [German Evidence-Based Medicine Network] e.V.).

Cohort study

Comparative observational study in which persons (cohorts) with or without an intervention/exposure (to which they were not assigned by the investigator) are observed over a defined period to detect differences in the occurrence of the target disease. Cohort studies can be prospective or retrospective (Deutsches Netzwerk Evidenzbasierte Medizin e.V.).

Case control study

Retrospective observational study in which a group of persons with a target disease ("cases") and a group of persons without the disease ("controls") are compared for the presence of exposure factors (risk or protective factors). Case control studies are suitable particularly for epidemiological medical questions regarding the etiology of rare diseases or rare treatment side effects (Deutsches Netzwerk Evidenzbasierte Medizin e.V.).

Retrospective study

In a retrospective study, the disease (the event) has already occurred at the start of the study and risk factors for the disease are sought retrospectively (Deutsches Netzwerk Evidenzbasierte Medizin e.V.).


  • Deutsches Netzwerk Evidenzbasierte Medizin e.V. (Hg.): Glossar zur Evidenzbasierten Medizin. Online: >>>