Table 1 Definition of expressions used throughout the article

Phase 1:

Initial phase

Study phase between initial invitation to the HaBIDS panel and invitation to the timely unlimited panel (includes questionnaires A to K)

“Mixed-mode: paper” group

group of individuals who chose to participate via paper-and-pencil questionnaires in the mixed-mode survey

“Mixed-mode: online” group

group of individuals who chose to participate via online questionnaires in the mixed-mode survey

“Online-only” group

group of individuals who participate in the online-only survey (participation only possible via online questionnaires)

Initial online participants

group of individuals who participate via online questionnaires in both surveys, i.e. union of the two groups “mixed-mode: online” and “online-only”

Phase 2:

Extended phase (online panel)

Study phase between invitation to the timely unlimited panel and latest questionnaire (includes questionnaires L to P)

Former paper participants

“mixed-mode: paper” who continued participating in the extended phase, i.e. who switched from participation via paper-and-pencil to online questionnaires

All-time online participants

online participants who continued participating in the extended phase

Withdrawers

participants who formally withdraw from HaBIDS

Discontinuers

participants who do not formally withdraw, but who are no longer filling in questionnaires (this condition is fulfilled if at least two consecutive questionnaires are missing and the participant does not return to the study; see for Additional file 4 that illustrates discontinuation)

Regular users

participants who neither withdraw nor discontinue