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Pijls et al. are to be congratulated on their analysis of the accuracy of the activation of a citizen rescue system for cardiac arrest [1]. Regarding their evaluation of the impact on survival, however, we would like to raise some methodological problems. Based on a positive correlation between the density of volunteer rescuers and the gain in survival among victims with and without responding citizens (with an increase from 16.0% to 34.8% in the municipalities with the highest volunteer density), the authors state: ‘These findings suggest that survival may even further increase with higher numbers of volunteers’. However, the methodology used is inappropriate to prove causality between higher volunteer density and higher survival and to estimate the magnitude of the impact of a higher attendance rate of alerted citizen responders. Indeed, the reasons why some victims are not reached by the alerted volunteers and why some municipalities have a low volunteer density are not explored. Probably, age, co-morbidity, socio-economic status, distance to the nearest emergency medical services (EMS) station, location of the collapse, etc. play a role. As these factors have some prognostic value, the presence of an alerted volunteer at the scene may largely be a marker for good outcome, and not a pivotal element in survival. Consequently, the impact on

How to evaluate the impact of a citizen rescue system on survival from cardiac arrest?