Table 1.

Definition of expressions used throughout the article

Expression	Definition
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
Both phases
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