questionnaire, they give the resear

questionnaire, they give the researchers written consent to use their information anonymously for research purposes. The survey was conducted from December 2009 to January 2011 and consisted of a baseline questionnaire and a follow-up questionnaire one year later. At baseline, all participants received a sealed envelope at their home address containing a postcard with an invitation to participate, a questionnaire survey and an incentive (lottery ticket for a national lottery) [30]. At follow-up, only those who had responded at baseline were sent a second postcard, a follow-up questionnaire and an incentive (lottery ticket). Completing the questionnaires took approximately 20 min. The participants were asked to fully complete and return the questionnaire within two weeks. One reminder containing a postcard was sent to all participants after one week.