Economize with mandatory fields, each mandatory field can be a reason why a participant does not like.
A good example of a meaningful maandaatory field is compliance questions or the confirmation of the terms
Radio buttons are machine readable and unambiguous (e.g., workshops, travel), use them as often as they can
Radio buttons can no longer be deselected once a selection has been made, but only a different selection can be selected. Therefore, if necessary, add an option to deselect (e.g., 'no workshop')
Do not turn off the query to participate, unless they are doing a survey and do not need participant management.
Avoid changes to the questionnaire as soon as the invitations have been sent and the first participants have answered. Yve always shows the current questions and options, not the historical values.
Add a text area as a comment field in which the participant can get rid of everything that the questionnaire did not cover