Odds Ratio (OR) measures the association between an outcome and a treatment/exposure. Or in other words, a comparison of an outcome given two different groups (exposure vs. absence of exposure).
OR is a comparison of two odds: the odds of an outcome occurring given a treatment compared to the odds of the outcome occurring without the treatment.
Odds represent the probability of an event occurring divided by the probability of an event not occurring.
Although related, probability and odds are not the same. Probability values can only range from 0 to 1 (0% to 100%), whereas odds can take on any value.
An OR value of 1 indicates no effect on the odds from the exposure to the outcome; of OR values less than 1 indicate that lower odds of the outcome are attributed by the exposure; and of OR values greater than 1 indicated that higher odds of the outcome are attributed by the exposure.
OR can be used to assess if a particular treatment is considered a ‘risk factor’ for a particular outcome.
To calculate OR, the frequencies of two dichotomous variables are required.
For example: a study consist 263 participants, where the aim of the study is to assess the OR of having the flu virus given the presence of a diet pill.
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