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Conventional metrics to quantify the magnitude of a treatment effect (i.e. measure of association) include the risk ratio or relative risk; relative risk reduction, odds ratio, number needed to treat, and hazard ratio (see below). Examples of calculating each are provided in Table 6.2 with corresponding examples from the LEADERS FREE trial.

Table 6.2 Common measures of association and calculations.

Measure of association	Calculation	LEADERS FREE example
Relative risk (RR)	Event rate in group 1/ Event rate in group 2	0.096/0.129= 0.74
Relative risk reduction (RRR)	1–RR	1–0.74 = 0.26
Odds ratio (OR)	(Prob of event in group 1/Probability of no event in group 1) / (Prob of event in group 2/Probability of no event in group 2)	(0.096/0.904) / (0.129/0.871) = 0.720
Number needed to treat (NNT)	1/Absolute risk reduction	1/(0.148–0.106) ~ 24

There is no “correct” singular metric to quantify a treatment effect. Often, it is recommended to incorporate several of these to appreciate both relative and absolute effects.