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Table 1 Characteristics of the included studies and of the number needed to treat (NNT)

From: Number needed to treat (NNT) in clinical literature: an appraisal

Characteristics Meta-analysis (n = 23) RCT (n = 17) Cohort (n = 9) Nested case–control (n = 2) Overall (n = 51)
Journal
JAMA 9 (39.1%) 4 (23.5%) 2 (22.2%) 2 (100.0%) 17 (33.3%)
Lancet 6 (26.1%) 7 (41.2%) 1 (11.1%) 0 (0.0%) 14 (27.5%)
Am J Med 2 (8.7%) 0 (0.0%) 2 (22.2%) 0 (0.0%) 4 (7.8%)
 Other 6 (26.1%) 6 (35.3%) 4 (44.4%) 0 (0.0%) 16 (31.4%)
Country
 USA 13 (56.5%) 2 (11.8%) 6 (66.7%) 0 (0.0%) 21 (41.2%)
 UK 4 (17.4%) 2 (11.8%) 0 (0.0%) 0 (0.0%) 6 (11.8%)
 Canada 1 (4.3%) 2 (11.8%) 1 (11.1%) 2 (100.0%) 6 (11.8%)
 Other 5 (21.7%) 11 (64.7%) 2 (22.2%) 0 (0.0%) 18 (35.3%)
Disease/condition
 Infections and infestations 4 (17.4%) 2 (11.8%) 1 (11.1%) 0 (0.0%) 7 (13.7%)
 Cardiac disorders 3 (13.0%) 3 (17.6%) 1 (11.1%) 0 (0.0%) 7 (13.7%)
 Psychiatric disorders 4 (17.4%) 3 (17.6%) 0 (0.0%) 0 (0.0%) 7 (13.7%)
 Other 12 (52.2%) 9 (52.9%) 7 (77.8%) 2 (100.0%) 30 (58.8%)
Primary outcome of study
 Efficacy 12 (52.2%) 16 (94.1%) 2 (22.2%) 0 (0.0%) 30 (58.8%)
 Safety 2 (8.7%) 1 (5.9%) 6 (66.7%) 2 (100.0%) 11 (21.6%)
 Efficacy and safety 9 (39.1%) 0 (0.0%) 1 (11.1%) 0 (0.0%) 10 (19.6%)
Type of variable (primary outcome)
 Binary 22 (95.7%)a 13 (76.5%) 5 (55.6%) 1 (50.0%) 41 (80.4%)
 Time to event 1 (4.3%) 4 (23.5%) 4 (44.4%) 1 (50.0%) 10 (19.6%)
Relative effect measure
 Yes           
 Relative risk 11 (47.8%)b 5 (29.4%) 2 (22.2%) 0 (0.0%) 18 (35.3%)a
 Odds ratio 9 (39.1%)b 4 (23.5%) 2 (22.2%) 1 (50.0%) 16 (31.4%)a
 Hazard ratio 1 (4.3%) 3 (17.6%) 3 (33.3%) 0 (0.0%) 7 (13.7%)
 Rate ratio 0 (0.0%) 0 (0.0%) 1 (11.1%) 1 (50.0%) 2 (3.9%)
 No 3 (13.0%) 5 (29.4%) 1 (11.1%) 0 (0.0%) 9 (17.6%)
Outcome expressed with NNT
 Primary outcome 6 (26.1%) 14 (82.4%) 7 (77.8%) 1 (50.0%) 28 (54.9%)
 Secondary outcome 0 (0.0%) 2 (11.8%) 0 (0.0%) 0 (0.0%) 2 (3.9%)
 Primary and secondary outcomes 17 (73.9%) 1 (5.9%) 2 (22.2%) 1 (50.0%) 21 (41.2%)
NNT for benefit or harm?
 Benefit 8 (34.8%) 15 (88.2%) 3 (33.3%) 0 (0.0%) 26 (51.0%)
 Harm 2 (8.7%) 1 (5.9%) 6 (66.7%) 2 (100.0%) 11 (21.6%)
 Benefit and harm 13 (56.5%) 1 (5.9%) 0 (0.0%) 0 (0.0%) 14 (27.5%)
Type of NNT calculated in the study
 Person-based NNT 21 (91.3%)a 13 (76.5%) 5 (55.6%) 1 (50.0%) 40 (78.4%)
 Person-time-based NNT 2 (8.7%) 4 (23.5%) 4 (44.4%) 1 (50.0%) 10 (21.6%)
Completeness of NNT estimate
 Control event rate
  Yes 13 (56.5%) 17 (100.0%) 6 (66.7%) 1 (50.0%) 37 (72.5%)
  No 10 (43.5%) 0 (0.0%) 3 (33.3%) 1 (50.0%) 14 (27.5%)
 Time horizon
  Yes 10 (43.5%) 17 (100.0%) 9 (100.0%) 2 (100.0%) 37 (72.5%)
  No 13 (56.5%) 0 (0.0%) 0 (0.0%) 0 (0.0%) 14 (27.5%)
 Confidence intervals
  Yes 16 (65.2%)c 8 (47.1%) 8 (88.9%) 1 (50.0%) 32 (62.7%)
  No 8 (34.8%) 9 (52.9%) 1 (11.1%) 1 (50.0%) 19 (37.3%)
  1. aThe variable for the primary outcome of one meta-analysis is binary, and pooled OR (95% CI) was calculated. However, a person-time-based NNT was calculated by taking the reciprocal of RD between pooled event rates per 1000 patient-years (Preiss 2011)
  2. bOne study reported relative risk (RR) and odds ratio (OR) (Maher et al. 2011)
  3. cConfidence interval was provided with NNT only for the primary outcome in a study reporting NNT for several outcomes (Green et al. 2007)