Aim
To increase survival after out-of-hospital cardiac arrest (OHCA) in Denmark, volunteer responders are activated through a smartphone application (HeartRunner app) to quickly locate an automated external defibrillator (AED) and assist with cardiopulmonary resuscitation (CPR). All dispatched volunteer responders who have been activated by the app receive a follow-up questionnaire to evaluate their participation in the programme. The content of the questionnaire has never been thoroughly evaluated. We therefore aimed to validate the content of the questionnaire.

Methods
Content validity was evaluated qualitatively. It was based on individual interviews with three experts, along with three focus group interviews and five individual interviews using cognitive interview technique, with a total of 19 volunteer responders. The interviews were also used to inform refinements of the questionnaire to reach improvements in content validity.

Results
The initial questionnaire consisted of 23 items. After the content validation process, the questionnaire consisted of 32 items; with the addition of 9 new items. Specifically, some original items were merged into one item or divided into separate items. Moreover, we revised the order of items, some sentences were rephrased or reworded, an introduction and headlines to different sections were added, and skip logic were incorporated to hide non-relevant items.

Conclusion
Our findings support the importance of validating questionnaires to ensure accuracy of survey instruments. Validation led to modifications of the questionnaire, and we propose a new version of the HeartRunner questionnaire. Our findings support the content validity of the final HeartRunner questionnaire. The questionnaire may allow the collection of quality data to evaluate and improve volunteer responder programmes.