# Table 3 Mortality assessed by TRISS and standardized mortality ratio

ground ambulance N = 5253helicopter N = 3524P value
Number of death387195
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
Z4.07.7
M0.870.87
SMR0.87 (0.80–0.93)0.67 (0.60–0.74)<0.001
1. SMR: standardized mortality ratio
2. 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
3. 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
4. Z score is the statistic compared with a standard normal distribution; null hypothesis is W = 0
5. M examine the similarity in the injury severities in the observed data compared to the predicted database
6. SMR = (number of observed death / number of expected death); Z-statistic, − 3.66 