Randomization
Stratification into subsamples and their size as well as randomization of each subsample are illustrated in Fig. The stratification and randomization were conducted by the National Institute of Public Health in collaboration with project members from the participating regions. Due to the timing of the different processes in conduction of the survey, the randomization was conducted before the initial split into (a) members registered to receive digital post and (b) members not registered to receive digital post. Hence, all eligible individuals were included in the randomization. As mentioned previously, it was only feasible to conduct the study among sample members who were registered to receive digital post. In the present study, 7.5% of the study sample were excluded as they were not registered to receive digital post. This exclusion is mainly an issue for subsample three (women aged 75 years or older), as the proportion who have unsubscribed digital post is high in this group (41.7%). The Fig. 