Odds Ratio


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. Therefore, if A is the probability of subjects affected and B is probability of subjects not affected, then odds = A /B.

Therefore, the odds of rolling four on a dice are 1/5 or 20%.

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.

Important points about Odds ratio:

  • Calculated in case-control studies as incidence of outcome is not known
  • OR >1 indicates increased occurrence of event
  • OR <1 indicates decreased occurrence of event (protective exposure)
  • Look at CI and P value for statistical significance of value
  • In rare outcomes OR = RR   (RR = Relative Risk). This applies when the incidence of disease is < 10%