Relative and Attributable Risks
Absolute Risk
• Involves people who contract disease due to an exposure
• Doesn’t consider those who are sick but haven’t been exposed
Relative Risk
Definition:
A measure of the strength of association based on prospective studies (cohort studies).
Relative Risk in Case-Control Studies
• Can’t derive incidence from case-control studiesBegin with diseased people (cases) and
non-diseased people (controls)
• Therefore, can’t calculate relative risk directly
• But, we can use another method called an odds ratio
When is the Odds Ratio a Good Estimate of Relative Risk?
• When cases are representative of diseased population
• When controls are representative of population without disease
• When the disease being studied occurs at low frequency
REMEMBER !!!
• An odds ratio is a useful measure of association
• In a cohort study, the relative risk can be calculated directly
• In a case-control study the relative risk cannot be calculated directly, so an odds ratio is used instead
OR, expressed as a proportion:
Attributable Risk for an Exposed Group (cont.)
From previous relative risk example:
Calculation for Proportional Incidence in Total PopulationFirst calculate A-R for
group from Formulas 11.1 & 11.2
(previous slide), then use Formula
11.3
For proportion of the incidence in the
total population, use Formula 11.4
Summary• Relative risk and odds ratio are important as
measures of the strength of associationImportant for deriving causal inference
• Attributable risk is a measure of how much disease risk is attributed to a certain exposureUseful in determining how much disease can
be prevented• Therefore:
Relative risk is valuable in etiologic studies of disease
Attributable risk is useful for Public Health guidelines and planning