In order to translate measures of relative risk, such as odds ratios or hazard ratios, to the more easily interpretable changes in absolute risk, we need baseline risks: the proportion of people in the ‘unexposed’ group who experience the outcome of interest. This will help make sense of the real magnitude of the risks reported in research and to be able to communicate its relevance to wider audiences.

Unfortunately, baseline risks are not readily obtained for case-control studies: a fairly common design in which people who have experienced the outcome are identified (the cases), and matched to some extent (say same age and sex) with people who have not experienced the outcome (the controls), and then retrospectively look at what proportion in each group had been exposed to the risk factor. This is the opposite to a prospective cohort design, in which exposed and unexposed people are followed up over a long period of time: a case-control study is quicker and more efficient for rare outcomes, but does require good historical data which is not subject to ‘recall bias’ (people who suffer a bad outcome may tend to report exposures that they have heard might be linked to their condition). …


David Spiegelhalter

Statistician, communicator about evidence, risk, probability, chance, uncertainty, etc. Chair, Winton Centre for Risk and Evidence Communication, Cambridge.

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