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

Calculating Excess Risk

Relative Risk

Definition:

A measure of the strength of association based on prospective studies (cohort studies).

Determining Relative Risk

Interpreting Relative Risk

Relative Risk Calculations

Relative Risk Calculations (cont.)

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

Odds Ratio in Prospective (Cohort) Studies

Odds Ratio in Case-Control Studies

Odds Ratio in Case-Control Studies (cont.)

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

Attributable Risk

Definition:

The amount of disease that can be attributed to a certain exposure.

Concept of Attributable Risk

Attributable Risk for an Exposed Group

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

Calculations for Attributable Risks (cont.)

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