Fourth, the findings of our study might not show causality but rather association. This study was conducted under the assumption that resuscitation efforts might be conducted equally on every OHCA patient. However, it was possible that some rescuers might avoid aggressive resuscitative efforts for some patients, such as elderly patients or patients with severe underlying disease. In addition, the quality or implementation standards of each resuscitative procedure might not always be equal among the rescuers, even though CPR was generally performed in accordance with the Japanese CPR guidelines. Therefore, the findings of this study might be affected by this potential bias in resuscitative efforts. To minimise such interference with the causal relationship, blinding information that might influence the motivation of rescuers may be necessary, but it may be impossible in actual practice.