The odds ratio: calculation, usage, and interpretation

Abstract

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. Significance statistics used for the OR include the Fisher's Exact Probability statistic, the Maximum-Likelihood Ratio Chi-Square and Pearson's Chi-Square. Typically the data consist of counts for each of a set of conditions and outcomes and are set in table format. The most common construction is a 2 × 2 table although larger tables are possible. As a simple statistic to calculate, [OR = (a × d)/(b × c)], it can be hand calculated in a clnic if necessary to determine the odds of a particular event for a patient at risk for that event. 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.