The rare disease assumption
Encyclopedia
The rare disease assumption is a useful mathematical assumption
in epidemiologic
case control studies where the hypothesis
tests the association between an exposure and a disease. It is assumed that, if the prevalence
of the disease is low, then the odds ratio
approaches the relative risk
.
Case control studies are relatively inexpensive and less time consuming than cohort studies
. Since case control studies don't track patients over time, they can't establish relative risk
. The case control study can, however, calculate the exposure-odds ratio, which, mathematically, approaches the relative risk as prevalence falls.
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in epidemiologic
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
case control studies where the hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
tests the association between an exposure and a disease. It is assumed that, if the prevalence
Prevalence
In epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...
of the disease is low, then the odds ratio
Odds ratio
The odds ratio is a measure of effect size, describing the strength of association or non-independence between two binary data values. It is used as a descriptive statistic, and plays an important role in logistic regression...
approaches the relative risk
Relative risk
In statistics and mathematical epidemiology, relative risk is the risk of an event relative to exposure. Relative risk is a ratio of the probability of the event occurring in the exposed group versus a non-exposed group....
.
Case control studies are relatively inexpensive and less time consuming than cohort studies
Cohort study
A cohort study or panel study is a form of longitudinal study used in medicine, social science, actuarial science, and ecology. It is an analysis of risk factors and follows a group of people who do not have the disease, and uses correlations to determine the absolute risk of subject contraction...
. Since case control studies don't track patients over time, they can't establish relative risk
Relative risk
In statistics and mathematical epidemiology, relative risk is the risk of an event relative to exposure. Relative risk is a ratio of the probability of the event occurring in the exposed group versus a non-exposed group....
. The case control study can, however, calculate the exposure-odds ratio, which, mathematically, approaches the relative risk as prevalence falls.