Two independent cross-sectional single stage interview based surveys were conducted in 2005 and in 2008. Both surveys were embedded in the respective year's Health Omnibus Survey, under the auspices of the South Australian Health Commission. The interviews were conducted by Harrison Health Research .
Sample selection and interview procedures
The sample selection and interview procedures were the same for each survey. Samples were selected from both metropolitan and rural areas. For the metropolitan sample in 2005 386, and in 2008 375 "collectors' districts" were selected from those used by the Australian Bureau of Statistics in the 2001 and 2006 census respectively . For the country samples, all towns of 10,000 or more in population size and a selection of towns of at least 1,000 people were surveyed. The collectors' districts were chosen according to their probability of selection proportional to size. Within each collector's district a starting point was randomly selected. From this starting point, using a pre-determined process based on a "skip" pattern of every fourth household, 10 dwellings were chosen. Only one interview was conducted per household or dwelling, and, where more than one resident was aged over 15 years, the respondent was the person whose birthday was last. The sample was a non-replacement sample, and up to six separate visits were made to interview the person chosen to take part.
The interview was piloted during August 2005 and August 2008, with 50 interviews. No formal reliability study was done, but 10% of each interviewer's work was selected at random, and the respondents re-contacted and a number of questions were asked of them, to ensure they had been interviewed as reported. Interviews were conducted from September through to 31st December 2005 and 12th February through 14th July in 2008 respectively.
The structured respondent-based interview comprised a range of health-related and demographic questions, including present height and weight. The eating disorder behavior and attitude questions were written by the first author (PH), and were modeled on related questions used in the investigator-based interview, the Eating Disorder Examination (EDE; ). The questions were embedded towards the end of the interview. Three eating disorder behaviors were assessed, namely binge eating, purging and strict dieting or fasting. Current regular use of these behaviors was defined as the behavior occurring at least weekly over the three months prior to the interview. In order to assess eating disorder attitudes respondents were asked about the importance of weight and/or shape to self-evaluation. A further question to investigate burden was asked namely assessing 'days out of role' . Body mass index (BMI; kg/m2) was calculated from self-reported weight and height.
The specific questions relating to eating disorders in the survey were: (i) I would now like to ask you about episodes of overeating that you may have had recently. By overeating, or binge eating, I mean eating an unusually large amount of food in one go and at the time feeling that your eating was out of control, [that is you could not prevent yourself from overeating, or that you could not stop eating once you had started]. Over the past three months how often have you overeaten in the way I have described? Responses were made from a 4-point list of
'not at all', 'less than weekly', 'once a week' and 'two or more times a week'; (ii) The next questions are about various weight-control methods some people use. Over the past three months have you regularly used, that is at least ONCE A WEEK, used any of the following: laxatives, diuretics (water tablets), made yourself sick, in order to control your shape or weight? Responses were either 'yes' or 'no'; (iii) Over the past three months have you regularly e.g. at least once weekly, or recurrently during the three months, done any of the following: gone on a very strict diet, or eaten hardly anything at all for a time, in order to control your shape or weight? Responses were either 'yes' or 'no'; (iv) In the past three months has your weight and/or your shape influenced how you think about (judge) yourself as a person? E.g. has it been a really important issue to you/to your self-confidence? Responses were on a 6-point scale from 'not at all' to 'extremely (the most important thing for you)' with a cut-off of ≥ 4 to indicate at least moderate importance; and in 2005 (v) During the past four weeks, on how many days (approximately), if any, were you unable to complete your work, study or household responsibilities because of any problem with your physical or emotional health? The number of days between 0 and 28 was recorded. In 2008 participants were also asked about subjective binge eating and health related role impairment i.e. ... have you felt your eating was out of control when others might not agree the amount of food was large (e.g. 2-3 pieces of bread) and have you been unemployed or out of work for health reasons?
Data were weighted by the inverse of the individual's probability of selection, then re-weighted to benchmarks derived from the Estimated Resident Populations at 30th June 1994 (and for the 2008 survey in 2006) by age, sex and Local Government Area, from the Australian Bureau of Statistics (Catalogue No 3204.4). The stratified cluster sampling approach was taken into account during the entire statistical analysis.
Numerical data were presented as mean values and standard deviations (SD). T-tests, Mann-Whitney U and chi-squared tests as appropriate were used to test for between group differences. Multivariable logistic regression analyses were conducted to determine the likelihood of a significant difference in features between indigenous and non-indigenous eating disorder behavior controlling for values of identified secular differences in each year of study. A significance level of 0.05 was employed for all tests. Analyses were conducted using the SPSS for Windows version 18.
All subjects in the study gave verbal informed consent to their participation and the study was approved as ethical by the Government of South Australia Department of Health. All participants received written information about the survey prior to consent being obtained. Written consent was deemed impractical in this large low risk survey by the Department. Verbal consent was obtained by the interviewers and audited by the Department, and the oral informed consent process was approved by the research ethics committee of the Department.