TABLE III.
Comparison of Crude, Regression, and Marginal Structural Model Estimates of Effect of Treatment Time on Mortality
| Estimate | >12 to 24 Hr | >24 to 48 Hr | >48 to 120 Hr | >120 Hr |
|---|---|---|---|---|
| Crude relative risk* | ||||
| Point estimate | 0.50 (0.18 to 1.01) | 1.13 (0.58 to 2.05) | 1.19 (0.67 to 2.03) | 1.17 (0.24 to 2.65) |
| P value | 0.06 | 0.69 | 0.52 | 0.77 |
| IPTW relative risk*† | ||||
| Point estimate | 0.45 (0.15 to 0.98) | 0.83 (0.43 to 1.44) | 0.58 (0.28 to 0.93) | 0.43 (0.10 to 0.94) |
| P value | 0.03 | 0.49 | 0.03 | 0.03 |
| Standardized risk ratio*‡ | ||||
| Point estimate | 0.47 (0.14 to 1.11) | 0.94 (0.44 to 1.76) | 0.58 (0.21 to 1.09) | 0.43 (0.09 to 0.94) |
| P value | 0.07 | 0.85 | 0.09 | 0.05 |
The point estimates are given with the 95% confidence intervals in parentheses. For all analyses, the t0 group (treated at twelve hours or less) was used as the referent group.
Inverse probability of treatment-weighted (IPTW) relative risk estimates were generated using a model for treatment assignment controlling for the subset of covariates with bivariate associations with mortality (Wm): New Injury Severity Score, Glasgow Coma Scale score, Northeast region, age, arrival time, the number of serious extremity/pelvic or head/neck injuries, the number of femoral fractures, the presence of cardiac or cerebrovascular comorbidities, teaching status, and American College of Surgeons level-1 designation.
The standardized risk ratio analysis involved the use of the same treatment model as inverse probability of treatment-weighted analysis but modified weights to give the estimated proportionate risk that would have been observed if the subjects in the early treatment group (group t0) had received treatment at a later time.