| ground ambulance *N* = 5253 | helicopter *N* = 3524 | P value |
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Number of death | 387 | 195 | |

Observed mortality, % | 7.4 (6.7–8.1) | 5.5 (4.8–6.3) | |

Expected mortality, % | 8.3 (7.8–8.8) | 8.2 (7.6–8.8) | |

W | + 1.1 (0.6–1.7) | + 2.7 (2.1–3.2) | <0.001 |

Z | 4.0 | 7.7 | |

M | 0.87 | 0.87 | |

SMR | 0.87 (0.80–0.93) | 0.67 (0.60–0.74) | <0.001 |

- SMR: standardized mortality ratio
- Expected mortality calculated with the formula = 1 / (1 + exp.(−b)), where b = − 0.4499 + (0.8085 * RTS) + (− 0.0835 * ISS) + (− 1.7430 * age) / b = − 2.5355 + (0.9934 * rts) + (− 0.0651 * ISS) + (− 1.1360 * age). Age < 55 years = 0 and age ≥ 55 = 1
- W is the difference between predicted number of survivors and the actual number of survivors, divided by the total number of cases divided by 100
- Z score is the statistic compared with a standard normal distribution; null hypothesis is W = 0
- M examine the similarity in the injury severities in the observed data compared to the predicted database
- SMR = (number of observed death / number of expected death); Z-statistic, − 3.66