Implementing GRADE: calculating the risk difference from the baseline risk and the relative risk

Abstract

A key step in implementing the GRADE (Grading of Recommendations Assessment, Development and Evaluation) system is the estimation of a risk difference based on estimates of the baseline risk and the relative risk estimated from different sources. In this paper we describe a simple and effective method to calculate confidence intervals (CIs) for the risk difference for this situation. Whenever an independent source is available to estimate the baseline risk for the population to which the effect estimates should be applied, this source should be used and CIs for the absolute risk difference should be calculated taking all sources of uncertainty into account.

Keywords: Epidemiology; Statistics & Research Methods.

Figure 1

Absolute risk reduction (expressed as events prevented per 1000 women) for effect of low-dose, low-molecular-weight heparin on venous thrombolic events, from Bates et al. Four 95% CIs are shown, (A) representing imprecision of relative risk (RR) only, (B) imprecision of baseline risk (BR) only, (C) a correct interval using MOVER-R and (d) calculated directly from upper and lower limits for both BR and RR.

Figure 2

Absolute risk difference (in %) for effect of low-dose bupivacaine on spinal anaesthesia efficacy, from Arzola and Wieczorek. Four 95% CIs are shown in figure 1.