Introduction
Epidemiology is the medical science interested in what causes disease and how it spreads among people; it studies the distribution and determinants of disease in human populations. Its main aim is to find and establish a causal relationship between an exposure to a risk factor and an outcome, and the main instruments for that are the epidemiological studies. This topic covers the observational designs, with their advantages and disadvantages.
Observational studies
- They do not involve any intervention; things take their natural course. Changes of one characteristic are studied in relation to another.
- They can be descriptive or analytical.
- They investigate the occurrence and distribution of diseases and provide useful information that may initiate further research into the problem.
Descriptive studies
They have no control or comparison group and involve writing up case reports and case series.
Case reports
- A detailed description and presentation of a single or several cases. Widely used in rare disease research, to present new diseases and syndromes or an unusual course of a common disease.
- Advantages: the only way of describing and publishing rare diseases and syndromes or an unusual course of a common disease; a source of hypotheses about aetiology, treatment and resolution of diseases that can later be evaluated by more complex epidemiological studies; provide very detailed and important information about pathogenesis, development and treatment; a bridge between laboratory and clinical research.
- Disadvantages: highly susceptible to bias because of the small number of cases, especially when commenting on treatment or survival; cannot be directly transferred to clinical practice because of the high level of uncertainty.
Case series
- A study of a larger group of patients with a single disease; a descriptive study, usually with no control or comparison group.
- Describes the clinical development and treatment of a group, usually gathered at one point in time.