![]() ![]() However, we can compare the odds of the use of Drug X in those who had no PONV vs. Since we select the outcome in both groups, we cannot calculate the relative chance (risk) of less PONV in the Drug X group because we do not know the chance of no PONV in the general population (who are not Drug X users), and therefore we have no comparison group. We use odds ratios instead, which can give us a measure of how strongly the risk factor is associated with the outcome.įor example, if we suspect that Drug X is associated with less PONV, then we could take 100 patients with out PONV to 100 patients with PONV and see how many in each group received Drug X. Our objective in such studies is to try to identify risk factors that are more strongly associated with one group than the other thus, risk and therefore relative risk cannot be calculated from these studies. In case-control studies, we already know what the outcome is and we separate groups into those with the outcome vs. To be able to calculate relative risk, we compare the risks of outcome in different groups. Odds ratios are used instead of relative risk for case-control studies. Therefore, the relative risk for PONV with Drug X vs. The probability of PONV with no Drug X is 40/100 or 0.40. The probability of PONV having received Drug X is 20/100 or 0.20. Therefore, the odds ratio for PONV with Drug X vs. The odds of PONV without Drug X is 40/60 or 0.67. The odds of PONV having received Drug X is 20/80 or 0.25. For example, lets say you want to compare the differences between PONV in women undergoing total abdominal hysterectomy receiving Drug X and those who do not, controlling for all other variables. Odds ratio: a ratio of odds in general they refer to the ratio of the odds of an event occurring in the exposed group versus the unexposed group. Note that this differs from risk (or probability): the risk of being on call is equal to (# of call days )/ (total # of days in a week) = 2/7 = 0.285. For example, if you are normally on call 2 out of 7 days in a week, then the odds of you being on call on a certain day of the week is = 0.40. Odds: the ratio of the probability that an event will occur versus the probability that the event will not occur, or probability / (1-probability). ![]()
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