Quality Assessment.

a Not selected because of potential overlap with Karamanos (2014).
b Several analyses described in the manuscript were eligible; the one with the smallest standard error of the estimated OR was selected.
c Study eligible based on quality criteria, but EMS-provider experience was “indeterminate” (see Table 3).
d Several analysis presented; first hour survival and first day survival data are adjusted, however the analysis with the outcome of main interest (hospital mortality) is not adjusted.
e Several analyses are presented, among which one matched and one adjusted analyses. Both of these analyses earned two stars for comparability, while the cohorts are not comparable in the crude analyses.
Newcastle-Ottawa Quality Assessment Scale
Selection:     1. Representativeness of the exposed cohort (prehospital intubation)
        2. Selection of the non-exposed cohort (no prehospital intubation)
        3. Ascertainment of exposure
    4. Demonstration that outcome of interest was not present at start of study
Comparability:     5. Comparability of cohorts on the basis of the design or analysis: most important factor
        6. Comparability of cohorts on the basis of the design or analysis: additional factors
Outcome:     7. Assessment of outcome
        8. Was follow-up long enough for outcomes to occur?
        9. Adequacy of follow up of cohorts
Cochrane Collaboration’s tool for assessing risk of bias: Domains
    A. Sequence generation
    B. Allocation concealment
    C. Blinding of participants and personnel
    D. Blinding of outcome assessors
    E. Incomplete outcome data
    F. Selective outcome reporting
    G. Other sources of bias
Quality Assessment.