Event rates and relative risk

It is straightforward to obtain the estimated probability of surviving for various key time points from the Kaplan-Meier estimates. In the Packer et al. (2001) example, the estimated survival probability at 12 months in the carvedilol group was 0.886 compared to 0.815 in the placebo group, an absolute difference of 7.1 per cent in the survival rates. A standard error formula provided by Greenwood (1926) enables us to obtain confidence intervals for these individual survival rates and for their differences. 

The estimated risk of dying in the first 12 months is then 0.114 in the cavedilol group compared to 0.185 in the placebo group. This enables the calculation of a relative risk at 12 months as 0.114/0.185 = 0.62 and the relative risk reduction is 38 per cent. Similar calculations can be undertaken at other time points. 

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