Prism reports the value more precisely as 2. You can interpret this odds ratio as a relative risk. The risk of a smoker getting lung cancer is about three times the risk of a nonsmoker getting lung cancer. Prism computes the confidence interval of the odds ratio using computed either using the Woolf logit method reference 1; the only method used by Prism 6 and earlier or the Baptista-Pike method 2 which we recommend. Choose on the Options tab of the Contingency table dialog.

Fagerland 3 reviews the various methods available to compute this confidence interval. If any cell has a zero and you choose the Woolf method, Prism adds 0. In this case, we suggest you switch to the Baptista-Pike method. Woolf B. On estimating the relation between blood group and disease. Ann Human Gene ; — Baptista J and Pike MC.

Recommended confidence intervals for two independent binomial proportions. Stat Methods Med Res.

## How to calculate odds ratios and probabilities in case-control studies? - Cross Validated

Odds ratio Here are the sample data for a case-control study the first study to link smoking to lung cancer. For example, enter numbers in the cells that would be consistent with a rare outcome and compare the OR and the RR. Then increase the frequency of the outcome by doubling, quadrupling, etc. Key Concept: Remember that in a cohort study you can calculate either a risk ratio or an odds ratio, but In a case-control study : you can only calculate an odds ratio.

In a cohort type study, one can calculate the incidence in each group, the risk ratio, the risk difference, and the attributable fraction. In addition, one can also calculate an odds ratio in a cohort study, as we did in the two examples immediately above. In contrast, in a case-control study one can only calculate the odds ratio, i.

### Statistics for the rest of us!

Consider once again the table that we used above to illustrate calculation of the odds ratio. In this table the total number of exposed and non-exposed subjects is not known, because sampling was done using a case-control design. One might find many or all of the cases in a source population, particularly if it is a reportable disease. In this example, the investigators found all thirteen cases, but then they just sampled 66 non-diseased people in order to estimate the exposure distribution in the source population.

When a case-control approach is used for sampling, we don't know how many exposed people it took to generate the 7 cases in the first row, and we don't know how many non-exposed persons were needed to generate the 6 cases in the second row. The information from non-diseased controls allow us to estimate the exposure distribution in the source population,. It was noted earlier in the module that the attributable proportion among the exposed in cohort type studies can also be calculated from the formula:.

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- Odds ratio.

Since the OR is an estimate of RR, then by analogy the attributable proportion among the exposed can be estimated in a case-control study from the formula:. Finally, since it is possible to estimate the attributable proportion in the exposed in a case-control study, it is also possible to compute the population attributable proportion in an analogous way to the computation in cohort type studies, i.

A study is done to examine whether there is an association between use of multivitamins and risk of coronary artery disease heart attacks. Compared to people who do not take vitamins, individuals who take daily multivitamins have a risk ratio of 0. Which of the following is a correct interpretation of this finding?

## Overview of Analytic Studies

The incidence of coronary artery disease in those who take multivitamins is 0. Those who take daily multivitamins have 0. The risk difference in this study is 70 per multivitamin users. The risk difference in this study is 30 per multivitamin users. The risk difference in this study is 0.

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A cohort study is conducted to investigate whether there is an association between alcohol consumption and esophageal cancer. The investigators group the subjects into four risk categories: a non-drinkers, b less than two drinks per week, c less than two drinks per day, d greater than two drinks per day.

### Your Answer

Which of the following statements is true regarding calculation of the strength of association in this study? Relative risk can't be calculated because there is more than one risk category. Risk difference can't be calculated because there is more than one risk category.