Odds Ratio

Time to read: 2 minutes


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 the probability of subjects not affected, then odds = A /B.

Therefore, the odds of rolling four on dice are 1/5 or an implied probability of 20%. [Note this is not the same as probability which would be 1/6 = 16.66%]

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 the incidence of outcome is not known
  • OR >1 indicates increased occurrence of an event
  • OR <1 indicates decreased occurrence of an event (protective exposure)
  • Look at CI and P-value for statistical significance of value (Learn more about p values and confidence intervals here)
  • In rare outcomes OR = RR   (RR = Relative Risk). This applies when the incidence of the disease is < 10%

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