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Table 1 Comparison of effect estimate by various subgroups

From: Seatbelt use and risk of major injuries sustained by vehicle occupants during motor-vehicle crashes: a systematic review and meta-analysis of cohort studies

Grouping variable Subgroup Numb. of studies Sample RR (95% CI) Prediction intervals Tau2 I2 (95%CI) H (95%CI) P-value for heterogeneity P-value Egger’s test P-value difference in subgroups
Overall (any injury) 11 38,662,538 0.47 (0.28–0.80) 0.07–3.27 0.669 99.9 (99.9–99.9) 34.7 (33.2–36.4) < 0.0001 0.361 NA
By injury site Head 6 32,964,587 0.49 (0.22–1.08) 0.03–8.65 0.910 99.3 (99.1–99.4) 11.6 (10.1–13.4) < 0.0001 0.046 0.090
Spinal 7 5,281,081 0.56 (0.37–0.84) 0.15–2.01 0.206 93.1 (88–96) 3.8 (2.9–5.0) < 0.0001 0.328
Facial 2 409,016 0.75 (0.40–1.43) 0.207 96.9 < 0.0001
Neck 2 855 0.69 (0.07–6.44) 1.761 56.6 0.129
Thoracic 2 855 0.96 (0.74–1.25) 0.0 0.0 0.754
Abdomen 2 4621 0.93 (0.84–1.04) 0.0 0.0 0.912
Upper limb 1 766 1.05 (0.83–1.34)
Lower limb 1 766 0.77 (0.58–1.04)
By crash type Motor vehicle collision 5 5,692,713 0.64 (0.46–0.89) 0.19–2.17 0.119 98.8 (98–99) 9.0 (7.4–10.8) < 0.0001 0.670 0.001
Rollover 3 3,276,677 0.43 (0.20–0.93) 0.0–963.4 0.465 99.2 (99–99.6) 11.4 (9.1–14.3) < 0.0001 0.268
Multiple 3 3447 0.37 (0.30–0.44) 0.11–1.27 0.0 0.0 (0.0–90) 1.0 (1.0–3.1) 0.506 0.025
Front 1 18,296,847 0.18 (0.18–0.19)
Side 1 8,571,748 0.15 (0.14–0.15)
Rear 1 2,821,106 0.09 (0.08–0.09)
By study sample size < median (6128) 5 4662 0.51 (0.20–1.30) 0.02–12.46 0.783 94.5 (90–97) 4.3 (3.2–5.8) < 0.0001 0.121 0.594
≥ median (6128) 6 38,657,876 0.45 (0.23–0.86) 0.04–5.10 0.659 99.9 (99.9–99.9) 48.7 (46.3–51.2) < 0.0001 0.340
By study type Hospital 3 6189 0.39 (0.01–1.72) 0.0–5025 1.079 61.1 (0–89) 1.6 (1.0–3.0) 0.077 0.038 0.967
Patient databases 8 38,656,349 0.48 (0.27–0.84) 0.06–3.88 0.648 99.9 (99.9–99.9) 40.6 (38.6–42.6) < 0.0001 0.452
Publication year Before 2010 3 5,270,440 0.36 (0.33–0.38) 0.22–0.57 0.0 0.0 (0.0–90) 1.0 (1.0–3.1) 0.530 0.036 0.269
On or after 2010 8 33,392,098 0.54 (0.30–0.98) 0.06–4.75 0.697 99.9 (99.9–99.9) 41.4 (39.4–43.4) < 0.0001 0.388
  1. NB: Absent predictive intervals could not be estimated due to inadequate number of studies (less than 3); absent I2 confidence intervals could not be estimated due to degrees of freedom less than 2 and Egger’s test was inestimable for groups with less than 3 studies