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Odds and Odds Ratios. By Lawrence Axten (here in spirit!). Odds. The odds of something happening can be referred to as the probability of an event happening. The odds of an event occurring are found by dividing the numbers of events by the number of non-events. For example….

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## Odds and Odds Ratios

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**Odds and Odds Ratios**By Lawrence Axten (here in spirit!)**Odds**• The odds of something happening can be referred to as the probability of an event happening. • The odds of an event occurring are found by dividing the numbers of events by the number of non-events.**For example…**• 51 boys are born out of every 100 births. • Therefore… Number of boys 51/number of girls 49=1.04 • Your odds of having a boy are 1.04. • If the number (in this case 1.04) is greater than one an event is more likely to happen. • If it is less than one it is less likely to happen. • The odds of an impossible event are zero.**Odds Ratio**• The odds ratio is found by dividing the number in the exposed or treated group by that of the control group. • Epidemiological studies generally try to identify factors that cause harm - those with odds ratios greater than one (event more likely). • Clinical trials look for treatments which reduce adverse events-so look for a odds ratio of less than one (less likely).**A point to note…**• Trials often quote the percentage reduction in odds ratio. • For example-the ISIS-4 trial reported a 7% reduction in the odds of mortality related to captopril. Not that the ratio was 0.93.**References.**• http://www.medicine.ox.ac.uk/bandolier/band25/b25-6.html • The Odds Ratio. J Bland. BMJ 2000 May; 320:1468.

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