Data collection
MAY
The data collection was conducted from February 2021 to May 2021. All invitations were distributed in the first week of February. The majority (91.3%) of the DNHS sample was invited via a secure electronical mail service (Digital Post) to complete a web questionnaire. Individuals who were not registered to use Digital Post (8,7%) were sent an initial invitation letter via the regular postal service, inviting them to complete a web questionnaire or the enclosed paper questionnaire (mixed-mode contact). Thus, the DNHS sample in 2021 was comprised of two subsets based on the mode of initial contact: (a) individuals invited initially by Digital Post, and (b) individuals invited via the regular postal service only. The total study sample in the present study consisted of 201,200 individuals from original national sample, Capital Region, Region Zealand and North Denmark Region whereof 15,087 (7.5%) had unsubscribed digital post. Hence, the eligible study population receiving a digital invitation was 186,133 individuals. The proportion who was not registered to use Digital Post varied from 0.8% for the age group 16–24 years (men: 0.9%; women: 0.7%) to 35.8% for the age group 75 years or older (men: 27.9%; women: 41.7%).A total of four reminders, excluding the initial invitation letter, were sent to the eligible population. If the web questionnaire was not completed after one week, a digital reminder was sent to non-response individuals. After yet another two weeks of non-response, these individuals were approached by a reminder letter sent by regular postal service. Enclosed in this letter was a paper questionnaire with identical content to the web questionnaire and a pre-paid return envelope. The remaining two reminders were sent by regular postal service, the last one with an enclosed paper questionnaire and a pre-paid return envelope. An overview of the reminder procedure in the digital path is given in supplementary material. The animation videos were part of the first digital invitation letter as well as the digital reminder for those groups randomized to receive a video (see details about randomization below and Fig. 
Illustration of the randomization into the control and intervention groups and sample size in the different groups (ns = number of individuals eligible for study; n = number of individuals in the different groups after exclusion of sample members who had unsubscribed digital post)