Box 3. Calculating measures of “effect” in the GATE frame

Risk ratio or relative risk [RR]
*Incidence or prevalence.
†The GATE checklists4 calculate 95% CIs for effect estimates.
‡NNE is the generic form of NNT (numbers needed to treat).
§When the CI for the risk difference crosses 0, the CI for an NNT (or NNE) passes through infinity.7
=  the ratio of occurrence* (risk) in the exposure group to the occurrence in the comparison group  =  EGO ÷ CGO (or EER ÷ CER in intervention studies)
Example: The occurrence (risk) of CHD death in the HRT group relative to the risk in the placebo group at the conclusion of HERS
RR =  12.5 ÷ 10.2 events per 1000 women per year (from box 1)  =  1.23 (95% CI 0.87 to 1.74)†
Risk difference [RD] or absolute risk difference [ARD]
=  the difference in occurrence* (or risk) between the exposure group and the comparison group  =  EGO – CGO or  =  EER – CER
Example: The difference in occurrence between the HRT treated group and the comparison group in HERS
RD =  12.5 – 10.2 events per 1000 women per year (from box 1)  =  2.3 events per 1000 women per year (95% CI −1.61 to 6.25)†
Number needed to expose [NNE] (or number needed to treat [NNT]) to reduce (or increase) events by 1 during time (T)
=  1 ÷ (EGO – CGO) or  =  1 ÷ (EER – CER)
Example: The number needed to be treated with HRT for 1 year to cause 1 additional CHD death in HERS
NNE or NNT‡
=  1 ÷ 2.3 events per 1000 women per year  =  435 women for 1 year (95% CI 160 to ∞ to −620)
ie, 435 women in HERS needed to be treated with HRT for 1 year to cause 1 additional CHD death