We recruited two groups, each consisting of at least 100 women, by random digit dialling fixed-line household numbers and mobile numbers. Women were eligible for the survey if they were aged 18-39 years and able to communicate verbally in English. Telephone numbers were randomly generated and dialled. If a fixed-line was answered, the respondent was asked whether there was an eligible woman in the household, and if so, she was invited to participate in the survey. For mobiles, if the call was answered, and the recipient was an eligible woman, they were asked to take part. The survey was 5-10 minutes long (for full questionnaire see additional file 1) and included questions on human papillomavirus (HPV) immunisation, past diagnoses of genital warts and chlamydia testing and treatment. The survey also sought information on whether the participant had a mobile phone if recruited by fixed-line, or if they had a fixed-line telephone at home if recruited by mobile.
For households contacted by fixed-line, preference was given to women aged 21-39 years over those aged 18-20, and if there were still more than one, random selection was used. Sampling was stratified by age, to recruit women 21-30 years old and 31-39 years in a ratio of 3:1, and state of residence, to recruit women in similar proportions to the geographic distribution of young women in Australia. Age stratification was required to ensure adequate power for pre-specified comparisons in the larger study. There was no pre-defined quota for women aged 18-20 years and they were included if their number was randomly dialled and no women aged 21-39 were available.
The selection process for fixed-line and mobile numbers was similar. Lists of currently active numbers were selected from the White Pages telephone book and modified by random terminal digit substitution to generate sets of numbers with proportional national distributions of number prefixes. Following number selection, a maximum of six calls were made to establish contact and a maximum of five more to complete the survey if there was an eligible person at the number. The survey was conducted using a standardised script and a Computer Assisted Telephone Interview (CATI) program. The outcome of every number called and responses to all survey questions were recorded. Both groups were surveyed by trained interviewers.
We compared call and survey responses between the two contact methods. Each number dialled was categorised as either 'eligible' (contact was in the target sex and age group), 'ineligible' (contact was incorrect sex or age, or had difficulty with English), 'unknown eligible' (answering machine/voicemail, no answer, refusal before eligibility determined) or 'invalid' (business, fax, disconnected). Two call response rates were calculated by dividing the number of interviews completed by: i) the number of 'eligible' calls; and ii) the number of 'eligible' calls plus a proportion of the 'unknown eligible' calls estimated to be eligible. The proportion of 'unknown eligible calls' estimated to be eligible was derived by using the population where eligibility was known and dividing all 'eligible' calls by all 'eligible' and 'ineligible' calls. For each main survey question we compared the response between the two groups (fixed-line and mobile) using logistic regression, adjusted for participant age. The study was approved by the University of New South Wales Human Research Ethics Committee.