Treatment Control Total Died within 30-days a b a + b Alive at 30-days c d c + d Total a + c b + d N = a + b + c + d A 2x2 contingency table + marginal totals
Measure Formula Example Number needed to treat (NNT) = 1 ARR 1 0.1 = 10 Equivalent to the average number of patients who need to be treated to prevent one additional event
low baseline risk RR = OR 1 − AR'()*+(, + 1 − AR'()*+(, OR Source: Grant, R. L. (2014). Converting an odds ratio to a range of plausible relative risks for better communication of research findings. BMJ, 348(4), f7450.
Relative effect: HR = 0.55 Absolute effect: ARR(12-months) = 20.0% 30.7% in the TAVI group 50.7% in the standard therapy group NNT(12-months) = 5 • HR uses all data at each time point • Not robust to departures from proportionality
Survival Survival HR = 3 HR = 1.5 1 2 Treatment A B Treatment A B Treatment arm Panel 1 (HR = 3) Panel 2 (HR = 1.5) A 1 1 B 0.9 0.5 Median survival times (years) Spruance SL et al. Hazard ratio in clinical trials. AntimicrobAgents Chemother 2004;48:2787–92.
both * Naylor et al. Measured enthusiasm: does the method of reporting trial results alter perceptions of therapeutic effectiveness? Ann Intern Med. 1992; 117(11):916-21.