Odds of an event happening is defined as the likelihood that an event will occur, expressed as a proportion of the likelihood that the event will not occur. Odds Ratio OR is a measure of association between exposure and an outcome. The OR represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure.
In a study examining the association between estrogen exposure and endometrial carcinoma outcome , the two by two table is shown below. The odds ratio OR is one of several statistics that have become increasingly important in clinical research and decision-making. It is particularly useful because as an effect-size statistic, it gives clear and direct information to clinicians about which treatment approach has the best odds of benefiting the patient.
Typically the data consist of counts for each of a set of conditions and outcomes and are set in table format. In addition to assisting health care providers to make treatment decisions, the information provided by the odds ratio is simple enough that patients can also understand the results and can participate in treatment decisions based on their odds of treatment success.
Event Happens a b. DOI: Categories: Lessons in biostatistics. PDF Article download : times. FB Share. General terms and conditions of use Cookies RPC. Journal Volume 19 June, Issue 2 The odds ratio: calculation, usage and interpretation.
The odds ratio: calculation, usage and interpretation. Mary L. There are a number of statistics that are helpful for making decisions about clinical interventions or drawing conclusions about effects of various substances or events in health-related situations, and one seen frequently is called the odds ratio.
The OR evaluates whether the odds of a certain event or outcome is the same for two groups. Specifically, the OR measures the ratio of the odds that an event or result will occur to the odds of the event not happening. Clinically, that often means that the researcher measures the ratio of the odds of a disease occurring or a death from a specific injury or illness happening to the odds of the disease or death not occurring. The odds ratio is used when one of two possible events or outcomes are measured, and there is a supposed causative factor.
The odds ratio is a versatile and robust statistic. For example, it can calculate the odds of an event happening given a particular treatment intervention 1. It can calculate the odds of a health outcome given exposure versus non-exposure to a substance or event 2. The clinical literature exhibits many instances of the odds ratio being used in research to estimate reduction in disease or disease complications if patients receive a particular drug or vaccine 3,4,5.
The odds ratio is a measure of effect size as is the Pearson Correlation Coefficient and therefore provides information on the strength of relationship between two variables. It is an indirect measure however, as will be seen in the section on interpretation of the statistic.
The calculation of the odds ratio is quite simple. The formula is as follows:. Another way to represent the formula is in table format:. For this particular example which can be generalized for all simple logistic regression models , the coefficient b for a two category predictor can be defined as.
Using the inverse property of the log function, you can exponentiate both sides of the equality [7a] to result in [6]:. In our particular example, e 1. Click here to report an error on this page or leave a comment. Your Name required. Your Email must be a valid email for us to receive the report! How to cite this page. Another example This example is adapted from Pedhazur How I published 35 papers during residency! Research — the way forward for society?
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