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Relative Risk

Discover the hidden truth behind relative risk and how it could impact your decision-making process in unexpected ways.

Relative Risk


The concept of relative risk is a fundamental concept in epidemiology and biostatistics. It is used to measure the strength of association between an exposure or risk factor and an outcome of interest. This guide aims to provide a comprehensive understanding of relative risk, its calculation, interpretation, and significance in medical research. It is particularly relevant for medical professionals preparing for the United States Medical Licensing Examination (USMLE).

What is Relative Risk?

Relative risk (RR), also known as risk ratio, is a statistical measure used to quantify the likelihood of an outcome occurring in an exposed group compared to an unexposed group. It is commonly used to evaluate the association between a specific risk factor or exposure and the development of a disease or outcome.

Calculation of Relative Risk

The relative risk is calculated by dividing the incidence of the outcome in the exposed group (E) by the incidence in the unexposed group (U):

[ RR = \frac{{\text{{Incidence in exposed group (E)}}}}{{\text{{Incidence in unexposed group (U)}}}} ]

Interpreting Relative Risk

The interpretation of the relative risk is as follows:

  • RR = 1: There is no difference in the risk of the outcome between the exposed and unexposed groups.
  • RR > 1: The exposed group has a higher risk of the outcome compared to the unexposed group. This suggests a positive association.
  • RR < 1: The exposed group has a lower risk of the outcome compared to the unexposed group. This suggests a protective effect or negative association.

Significance of Relative Risk

Relative risk is a valuable measure in medical research for several reasons:

  1. Assessing causality: A high relative risk indicates a strong association between the risk factor and the outcome, suggesting a potential causal relationship.
  2. Comparing interventions: Relative risk allows for a direct comparison of the effectiveness of different interventions or treatments in reducing the risk of an outcome.
  3. Identifying high-risk groups: Relative risk can help identify groups or populations at higher risk for a specific outcome, allowing for targeted interventions and preventive strategies.

Limitations of Relative Risk

While relative risk is a useful measure, it is important to consider its limitations:

  • Association vs. causation: Relative risk only measures the strength of association and does not establish causality. Other factors may be influencing the observed relationship.
  • Confounding variables: Potential confounding variables, which were not accounted for, may affect the relative risk.
  • Rare outcomes: Relative risk may overestimate the magnitude of risk for rare outcomes, as the numerator and denominator may be small.

Example Application

Consider a study investigating the association between smoking and the development of lung cancer. The study compares two groups: smokers and non-smokers. The incidence of lung cancer is 10% among smokers (E) and 2% among non-smokers (U).

[RR = \frac{{0.10}}{{0.02}} = 5]

Interpretation: Smokers have a five times higher risk of developing lung cancer compared to non-smokers.


Relative risk is a key measure used to assess the association between an exposure and an outcome in medical research. Its calculation and interpretation are essential for understanding the impact of risk factors on disease occurrence. By grasping the concept of relative risk, medical professionals can critically analyze research studies and make informed decisions in clinical practice.

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