Odds Ratio
Odds Ratio
##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%
CLINICAL EXAMPLE AND CALCULATION
In a study examining the association between estrogen (exposure) and endometrial carcinoma (outcome), the two by two table is shown below.
Learning point: In a two by two table, for ease of calculation ensure that the outcome of interest is always at the top and the exposure on the left.
annotations:
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