U01.06.007 Relative risk

Overview Relative Risk (RR) is a key measure used in epidemiological studies, particularly cohort studies, to compare the risk of developing a disease between exposed and unexposed groups.

Calculation of Relative Risk

Relative Risk is calculated as: RR=Risk in exposed groupRisk in unexposed group\text{RR} = \frac{\text{Risk in exposed group}}{\text{Risk in unexposed group}}RR=Risk in unexposed groupRisk in exposed group

Interpretation of Relative Risk

RR = 1: No association between exposure and disease.
RR > 1: Exposure is associated with an increased risk of disease occurrence.
RR < 1: Exposure is associated with a decreased risk of disease occurrence.

Example Calculation Consider a scenario where 5 out of 10 people exposed to radiation develop cancer, while 1 out of 10 people not exposed to radiation develop cancer. The calculation of RR would be: RR=5/101/10=5\text{RR} = \frac{5/10}{1/10} = 5RR=1/105/10=5 This indicates that people exposed to radiation have a 5 times greater risk of developing cancer compared to those not exposed.

Relative Risk vs. Odds Ratio

For rare diseases, where the prevalence is low, the Odds Ratio (OR) can approximate the Relative Risk. This is useful when working with case-control studies where direct calculation of RR might not be feasible.

Points to Remember

RR is primarily used in cohort studies to determine the strength of an association between exposure and disease.
RR > 1 indicates a positive association, suggesting exposure increases disease risk.
RR < 1 suggests a protective effect of the exposure against the disease.
For rare diseases, OR approximates RR, making it a useful measure in case-control studies.

Table: Relative Risk Interpretation

Relative Risk (RR)	Interpretation
RR = 1	No association between exposure and disease
RR > 1	Increased risk of disease with exposure
RR < 1	Decreased risk of disease with exposure

Table: Relative Risk Interpretation

Bibliography

Szklo, M., & Nieto, F. J. (2019). Epidemiology: Beyond the Basics. Jones & Bartlett Learning.
Rothman, K. J., Greenland, S., & Lash, T. L. (2008). Modern Epidemiology (3rd ed.). Lippincott Williams & Wilkins.
Hennekens, C. H., & Buring, J. E. (1987). Epidemiology in Medicine. Little, Brown and Company.

USMLE Step 1 Review Series

Curriculum

U01.06.007 Relative risk

Calculation of Relative Risk

Interpretation of Relative Risk

Relative Risk vs. Odds Ratio

Points to Remember

Table: Relative Risk Interpretation

Bibliography

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