Suggested answers to exercise: Cough during the day or at night and history of bronchitis, 5

Question 5: Why does SPSS put the words 'For cohort' in the Risk Estimate table?

Suggested answer

To calculate the relative risk of cough given a history of bronchitis, we need to calculate the risk of cough for children with a history of bronchitis and the risk of cough for children without a history of bronchitis. We then divide one by the other. We can only calculate these risks if we start with a group of people with a history and another group without a history, i.e. a cohort of people followed over time or a cross-sectional study where we obtain both current symptoms and history from the same sample. We cannot do it if we start with a group of children with cough and a second group of children who do not have cough, then find out whether they had a history of bronchitis. This would be a case-control study.

The relative risk is only interpretable if we have a cohort study, cross-sectional study, or clinical trial, not if we have a case-control study. The authors of SPSS are reminding us of this.

Having said that, the output is pretty confusing.

In some circumstances, we can estimate the relative risk in a case-control study using the odds ratio, but that is beyond the scope of this module.

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Last updated: 21 November, 2006.

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