During the recruitment of the panel, respondents who agreed to participate in the panel received a confirmation email, and a letter with login code. With the login code provided they could confirm their willingness to participate and immediately start the first questionnaire. This confirmation procedure, following the consent to participate given to the interviewer, ensured the double consent of each respondent to become a panel member and participate in the monthly panel questionnaires. The present study was part of the normal monthly panel questionnaires, for which no specific consents were asked after the general consent for panel participation was given.
It has to be noted that ethics approvals for questionnaire research among adults are not required in the Netherlands. In general, CentERdata abides by the Dutch "protection of personal data act" ("Wet Bescherming Persoonsgegevens", WBP), which is consistent with and derived from European law (Directive 95/46/EC).