Table 2 Modified Downs and Black quality scoring system

Reporting

Is hypothesis/objectives/aim clearly described (1)

Are the main outcomes to be measured clearly described in the introduction/methods (1)

Are the characteristics of the study/population clearly described (1)

Is the distribution of confounders among study groups clearly described and did the study adjust for those confounders (1)

Are the main findings described (1)

Have characteristics of patients lost to follow-up/did not complete the study been described (1)

External validity

Are study participants representative of entire population from which they were recruited (1)

Are study participants representative of current OHCA population (1)

Were those participants who were prepared to participate representative of the entire population from which they were recruited (1)

Internal validity

Was an attempt made to blind those measuring main outcomes of the study (1)

Did the study adjust for different lengths of follow-up of participants (1)

Were statistical tests used appropriate (1)

Was the outcome measure accurate (1)

Were the participants in the different groups recruited from the same populations (1)

Were study participants in the different groups recruited over the same time period (1)

Was there adequate adjustment of confounding in the analyses (1)

Study design

Cross sectional (0.5)

Cohort prospective (2)

Cohort retrospective (1)

Control group (yes = 1 point, no = 0 points)

Secondary/retrospective analysis (1)

Sample size adequate/power analysis (yes = 1, no = 0)

Total max points for study design (4 points)

Total points (20)