Floating absolute risk: an alternative to relative risk in survival and case-control analysis avoiding an arbitrary reference group.

We discuss the problem of describing multiple group comparisons in survival analysis using the Cox model, and in matched case-control studies. The standard method of comparing the risk in each group with a baseline group is unsatisfactory because the standard errors and confidence limits relate to correlated parameters, all dependent on precision within the baseline group. We describe the construction of standard errors for the parameters of all groups, without the need to select a baseline group. These standard errors can be regarded as relating to roughly independent parameters, so that groups can be compared efficiently without knowledge of the covariances. The method should assist in graphical presentation of relative risks, and in the combination of results from published studies. Two examples are presented.