A painting of a short, black haired woman with a red and white shirt on reading a green book. Art is Woman Reading by Rik Wouters.

Odds Ratio (OR) is a fundamental concept in epidemiology and statistics, providing a measure of association between exposure and outcome across different groups. This concept is pivotal in fields like medicine, public health, and social sciences, where understanding the relationship between risk factors and outcomes can guide critical decisions. This edition of Epi Explained delves into the intricacies of Odds Ratio, elucidating its definition, calculation, and interpretation, supplemented by historical context, real-world applications, and an example problem to enhance comprehension.

Introduction to Odds Ratio

The Odds Ratio is a measure that compares the odds of an outcome occurring in the presence of a particular exposure, to the odds of the same outcome occurring without that exposure. It’s a metric that helps researchers understand the strength and direction of the association between two binary variables.

Historical Context and Importance

The concept of odds and their ratios has been around for centuries, primarily in gambling contexts. However, its application in epidemiology and statistics gained prominence in the 20th century as researchers sought quantifiable measures to investigate the link between potential risk factors and disease outcomes. The OR became especially crucial in case-control studies, where direct risk estimation is not feasible due to the study’s retrospective nature.

Mathematical Definition

The odds of an event is defined as the probability of the event occurring divided by the probability of the event not occurring. Mathematically, if [math] P [/math]

is the probability of an event, the odds is given by: [math]{\text{Odds} = \frac{P}{1 – P}}[/math]


Simply put, this equation reads as “Odds can be calculated as the probability of an event occurring [math]{(P)}[/math], divided by the probability of the event not occurring [math]{(1 – P)}[/math]”.


The Odds Ratio (OR) is then calculated as the ratio of two odds:


[math]{\text{Odds Ratio (OR)} = \frac{\text{Odds of exposure in cases}}{\text{Odds of exposure in controls}} = \frac{A/C}{B/D} = \frac{A \times D}{B \times C}}

Calculation of Odds Ratio

To calculate the OR, data is often arranged in a 2×2 contingency table based on two binary variables: exposure (yes/no) and outcome (yes/no).


Outcome PositiveOutcome NegativeExposedabNot Exposedcd

It is then fed into the above equation to calculate a value. If your value is higher than 1, then those in an exposed condition have higher odds of developing the outcome, and the reverse is true if the value is between 0 and 1. Do keep in mind though that this does not mean that the odds are significantly higher or lower, as that depends on how you are calculating significance.

Interpretation of Odds Ratio

  • OR = 1: No association between exposure and outcome.
  • OR > 1: Positive association; exposure is associated with higher odds of the outcome.
  • OR < 1: Negative association; exposure is associated with lower odds of the outcome.


Practice Problem

Consider a study investigating the association between smoking (exposure) and lung cancer (outcome). The data might look something like this:


CancerNo CancerSmokers100200Non-Smokers30270

Get the Odds Ratio for the above table.

  • A. 3.5
  • B. 0.22
  • C. 18
  • D. 4.5
Answer Key, click to reveal

D. 4.5


This OR of 4.5 suggests that smokers have 4.5 times the odds of developing lung cancer compared to non-smokers.


Limitations and Considerations

While the OR provides valuable insights, it has limitations. It can overestimate the risk, especially for common outcomes. Moreover, it does not imply causation and can be influenced by confounding variables. Understanding the context and design of the study is crucial for accurate interpretation.



The Odds Ratio is a versatile and powerful statistical tool in epidemiological studies, offering insights into the relationship between exposures and outcomes. Its calculation and interpretation require careful consideration of study design and context.In this edition of Epi Explained, we have:

  • Defined Odds Ratios.
  • Learned how to calculate Odds Ratios.
  • Attempted a Practice Problem.

Come by again next week for another edition of Epi Explained.


Humanities Moment

This articles featured artwork is Woman Reading (1913) by Rik Wouters (Belgian, 1882 – 1916). Wouters, though short lived (dying at the age of 34 due to illness during World War 1), created over 200 paintings and sculptures and is known for a simplistic, straightforward style with warm colours and simple motifs. This simplistic and elegant approach also tended to shy away from any symbolism within his art; What you see is what you got, without the need of some grand narrative.

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