Assessment of study quality
| Study | Study design | Newcastle Ottawa scoring | Score (9) | Risk of bias | ||
| Selection (4) | Comparator (2) | Outcome (3) | ||||
| Cothren et al22 | Retrospective cohort | **** | – | – | 4 | High |
| Snow et al13 | Retrospective cohort | **** | – | * | 5 | High |
| Biffl et al24 | Retrospective cohort | *** | – | * | 4 | High |
| Wei et al11 | Retrospective cohort | *** | – | * | 4 | High |
| Miller et al6 | Retrospective cohort | *** | – | * | 4 | High |
| Stein et al19 | Retrospective cohort | **** | – | *** | 7 | Moderate |
| Cothren et al21 | Prospective cohort | ** | – | *** | 5 | Moderate |
| Malhotra et al20 | Prospective cohort | **** | – | * | 5 | High |
| Wagenaar et al10 | Retrospective cohort | **** | – | *** | 7 | Moderate |
| Callcut et al16 | Retrospective cohort | **** | ** | ** | 8 | Low |
| Burlew et al14 | Retrospective cohort | ** | – | * | 3 | High |
| DiCocco et al17 | Retrospective cohort | **** | – | *** | 7 | Moderate |
| Miller et al23 | Prospective cohort | **** | – | * | 5 | High |
| Biffl et al2 | Retrospective cohort | **** | – | * | 5 | High |
| Cothren9 | Retrospective cohort | **** | – | ** | 6 | Moderate |
| Biffl et al7 | Retrospective cohort | *** | – | ** | 5 | Moderate |
| Lebl et al15 | Retrospective cohort | **** | – | * | 5 | High |
| Hwang et al18 | Retrospective cohort | **** | – | * | 5 | High |
| Catapano et al5 | Retrospective cohort | **** | – | ** | 6 | Moderate |
Low risk of bias: 3 or 4 stars in selection domain and 1 or 2 stars in comparability domain and 2 or 3 stars in outcome/exposure domain.
Moderate risk of bias: 2 stars in selection domain and 0, 1, or 2 stars in comparability domain and 2 or 3 stars in outcome/exposure domain (modified to reflect the often appropriate omission of regression based on number of outcomes).
High risk of bias: 0 or 1 star in selection domain or 0 star in comparability domain or 0 or 1 star in outcome/exposure domain.