The odds ratio: Principles and applications

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Author: Aakshi Kalra
Date: January-April 2016
Publisher: Medknow Publications and Media Pvt. Ltd.
Document Type: Report
Length: 1,475 words
Lexile Measure: 1290L

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Byline: Aakshi. Kalra

The odds ratio (OR) is a simple tool, widely utilized in clinical research. As a simple statistic, it can be hand calculated to determine the odds of a particular event or a disease, and the information provided can be useful for understanding the results of a treatment/intervention. This article discusses the application of OR with examples and shows a simple way of performing the test using an online calculator.

The odds ratio (OR) is a measurement of association which compares the odds of disease or an event of those exposed to the odds of disease/event in those unexposed. It serves to determine the relation between exposure and outcome.

OR is a numeric expression of the strength of association between cause and an effect when both are expressed as categorical variables. As a rule of thumb, larger the OR, more will be the effect on the outcome.

The odds of an outcome are the ratio of occurrences in the presence of an event to the occurrences in the absence of an event. While odds can be calculated for any event, odds raitio is always with reference to a two different outcomes (like cases and controls).

We wish to answer a question like “what is the likelihood of a person to face an outcome, when a medical condition is present, compared to those where the medical condition is not present.” This is then answered as the ratio of the odds of outcome under the two conditions.

The OR can be used to evaluate whether the odds of an event is same for two groups in a study sample. The OR measures the ratio of the odds that an outcome will occur given a particular exposure to the odds of the outcome occurring in the absence of the exposure.[sup][1],[2],[3],[4]

The OR can also be a measure of the effect size.


*OR = (odds of disease in exposed)/(odds of disease in the nonexposed)

A contingency table for the OR is created as shown in [Table 1].{Table 1}

The OR is calculated as the odds of being exposed if a case (a:c) divided by the odds of being exposed if a control (b:d) and by transformation, can be depicted...

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Gale Document Number: GALE|A488635596