Risk B has the strongest association [Ref: http://www.thedoctorsdoctorcom/labtests/statistics.htm Odds ratio is a measure of the strength of the association between risk factor and the outcome. It is used to assess the risk of a paicular outcome (or disease) if a ceain factor (or exposure) is present. The odds ratio is a relative measure of risk, telling us how much more likely it is that someone who is exposed to the factor under study will develop the outcome as compared to someone who is not exposed. Odds ratio is the ratio of the odds that a case was exposed to the odds that a control was exposed. Odds ratio = = It can vary from 0 to infinity. For example: The data in the table below is information about infant bih weights and moality among white infants in New York City in 1974. Dead Alive Total Low bih wt. 618 4,597 5,215 Norm bih wt. 422 67,093 67,515 Total 1,040 71,690 72,730 The odds ratio for death in one year is OR= This odds ratio illustrates that moality is far more likely in the low bih weight group Internreating Odd's Ratios In order to interperate the odds ratio we need to calculate a confidence interval. We can say that a proposed risk factor acts as a significant risk to disease if the odds ratio is greater than one and the lower bound of the confidence interval does not include 1. Confidence interval The 95% confidence interval (or 95% confidence limits) would include 95% of results from studies of the same size and design in the same population. This is close but not identical to saying that the true size of the effect (never exactly known) has a 95% chance of falling within the confidence interval. If the 95% confidence interval for a relative risk (RR) or an odds ratio (OR) includes 1, then this is taken as no evidence of an effect. The practical advantages of a confidence interval (rather than a P value) is that they present the range of likely effects. Odds Ratio 95% confidence interval does not include 1 Interpretation 1 Yes No association >1 Yes Positive association between exposure and outcome at the 5% significance level (the odds of exposure is greater in cases than in controls) <1 Yes Negative association between exposure and outcome at the 5% significance level (the odds of exposure is smaller in cases than controls) 95% confidence interval does include 1 Association of exposure and outcome is not proved by the study at the 5% significance level An odds ratio of 1 indicates that a person with the disease is no more likely to have been exposed to the risk factor than is a person without the disease, suggesting that the risk factor is not related to the disease. An odds ratio of 1 implies that the event is equally likely in both groups. An odds ratio of less than 1 indicates that a person with the disease is less likely to have been exposed to the risk factor than is a person without the disease, implying that the proposed risk factor may actually be a protective factor against the disease. An odds ratio of more than 1 indicates that the person with disease is more likely to have been exposed to the risk factor than a person without the disease, meaning there is association between the disease and the risk factor.