Møller SG, Wissenberg M, Møller-Hansen S, et al. - As factors including differences in definitions and reporting may be responsible for the reported regional variation in incidence and survival after out-of-hospital cardiac arrest (OHCA), researchers here examined the regional differences in Denmark. The Danish Cardiac Arrest Registry yielded data of 12,902 patients with OHCA of presumed cardiac cause from 2009 to 2014. Of these patients, 1,550 (12.0 %) were alive 30 days after OHCA. OHCA incidence varied from 32.9 to 42.4 per 100,000 inhabitants; 30-day survival percentages varied from 8.5% to 13.8% and the number of survivors per 100,000 inhabitants varied from 3.5 to 5.9, across the regions. The five regions of Denmark differed in incidence and 30-day survival after OHCA. They recommend addressing the number of survivors of the background population in addition to addressing survival percentages in comparisons of survival, as inclusion bias can influence survival outcomes.