Fig. 1

Survey response rate calculation using American Association for Public Opinion Research guidelines. The survey response rate was 64 %. This minimal response rate calculation includes sample members who completed the survey (I), over all invited sample members, including all category (U) sample members for whom it is unknown whether the survey invitation reached them. These sample members are included in the denominator of the response rate because their contact details were confirmed and updated prior to survey implementation. It is estimated therefore that the majority of category (U) sample members received the survey invitation but chose not to respond