Resuscitation
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Previous research demonstrates that results from observational research correlate well with results from clinical trials, and if the former are well designed these can guide clinical practice. Observational studies in cardiac arrest research are beset by confounding due to illness severity and comorbidity. We aimed to count the number of studies that utilize comorbidity and illness severity scores and indices, and to measure the change in results across analyses that adjust for scores and indices. ⋯ A small proportion of cardiac arrest studies account for illness severity and comorbidity with scores and indices, and such adjustment tend to drive estimates towards the null (no difference in groups being compared). Confounding by illness severity and comorbidity is a significant source of bias in non-randomized cardiac arrest studies.