Video created: May 7, 2013
Knowing how to interpret an odds ratio (OR) allows you to quickly understand whether a public health intervention works and how big an effect it has. For example, how effective is the flu vaccine in preventing people from getting the flu?
Using hypothetical data, How to Calculate an Odds Ratio shows how an OR helps determine, on average, how many people who got the flu shot came down with the flu, versus the number of people who did not get the flu shot.
The video explains how to calculate and interpret an OR, and decide whether it indicates a positive or negative outcome. An OR of “1” would mean that the flu shot made no difference. So, if the outcome is something we were trying to increase, such as getting the flu shot in the first place, a positive outcome would be indicated by an OR of greater than 1. But, if the intervention is intended to decrease something, such as getting sick with the flu, an OR of less than 1 would show a positive outcome.
It’s possible to calculate an Odds Ratio or Relative Risk from the same data. However, these statistics are used in different circumstances. “Calculation of risk requires the use of “people at risk” as the denominator. In retrospective (case-control) studies, where the total number of exposed people is not available, RR cannot be calculated and OR is used as a measure of the strength of association between exposure and outcome. By contrast, in prospective studies (cohort studies), where the number at risk (number exposed) is available, either RR or OR can be calculated.”
Ranganathan, P., Aggarwal, R., & Pramesh, C. S. (2015). Common pitfalls in statistical analysis: Odds versus risk. Perspectives in clinical research, 6(4), 222.
The National Collaborating Centre for Methods and Tools is funded by the Public Health Agency of Canada and affiliated with McMaster University. The views expressed herein do not necessarily represent the views of the Public Health Agency of Canada.
NCCMT is one of six National Collaborating Centres (NCCs) for Public Health. The Centres promote and improve the use of scientific research and other knowledge to strengthen public health practices and policies in Canada.
