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 compares the odds of an outcome with and without 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 shows no effect, while OR < 1 indicates lower odds and OR > 1 indicates higher odds due to exposure.
OR assesses whether a treatment is a risk factor for an outcome.
Calculating OR requires the frequencies of two dichotomous variables.
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.