The North West Adelaide Health Cohort Study (NWAHCS) recruited between 2000 and 2002 with a total of n = 4060 adults participating. Randomly selected telephone numbers listed in the relevant postcodes (that equated to the boundaries of the suburbs selected to be included in the study) were drawn from the most current Electronic White Pages. A letter of invitation to participate was sent to these households followed within 10 days by a telephone call from trained health study recruiters. A randomly selected adult within the household (those with the next birthday aged 18 years and older) was asked to participate in the study. At each appointment, the participant was given additional detailed information about the study and asked to sign consent forms for participation in the study. The information given highlighted the longitudinal nature of the study, and participants were informed that they may be invited to participate in health-related sub-studies. Prior to the study commencing, approval for the research was obtained from the North West Adelaide Health Service Ethics of Human Research Committee.
Appointments were made for participants in one of the two hospital-based clinics in the region and participants were sent an information folder that included a questionnaire with questions on chronic disease, alcohol consumption, physical activity levels, quality of life and socio-economic details (including highest education level, marital status, work status, country of birth and household income level). Age, sex, smoking status, height, weight, and ever being told they had high blood pressure or high cholesterol were asked in the recruitment telephone interview. At the clinic a range of assessments were made including taking blood (to test fasting plasma glucose, lipids, HbA1c), skin prick tests to common allergens and spirometry lung function tests.
The overall response rate of the completed telephone interview, self-completed questionnaire and clinic biomedical assessment (including blood sample) was 49.6% (69% of those interviewed). This paper assesses data associated with the respondents who completed all aspects of the study. Full details of the methodology have been previously published [18–20].
To examine the representiveness of the NWAHS sample with regard to age, sex, area of residence and socio-economic status, a comparison was made using Australian Bureau of Statistics (ABS) Census figures. Socio-economic status was measured using the Socio Index for Areas, Index of Relative Social Disadvantage (SEIFA IRSD) .
To compare the other demographics and social characteristics of the respondents and the population estimates of key health risk factors, a comparison against a population-based survey, the South Australian Surveillance and Monitoring System (SAMSS), was undertaken. SAMSS is a representative, on-going, population household telephone interview surveillance/survey of the South Australian population based on EWP sampling and has operated each month since July 2002 using a consistent methodology . This involves a random sample of SA households with one person selected at random in each household according to next birthday. Trained health interviewers interview respondents using computer assisted telephone interviewing (CATI) technology and there is no replacement for non-respondents. From July 2002 to June 2004, n = 2904 adults in the NW suburbs of Adelaide were interviewed providing a non-replacement response rate of 68.7%. To compare physical activity rates, data from the South Australian Health Monitor were used. Methodology of this CATI survey, operated three times a year, is similar to SAMSS and has been detailed elsewhere . This is a separate comparable survey with a separate sample.
While the questions asked in NWAHCS and SAMSS were identical for age, sex, country of birth, household income, alcohol consumption, height and weight (to calculate body mass index (BMI)), current high blood pressure, current high cholesterol, physical activity and self-reported health status there were slight differences in wording of the question for highest education level, marital status, work status variables and smoking status. Questions on height, weight, blood pressure and cholesterol were only asked of the second half of the respondents although measurements in the clinic were undertaken on all participants.
All analyses were limited to data on respondents aged 18 years and over in the same geographical area to correspond to the NWAHCS sample. Data were weighted by age, sex, region and probalility of selection within the household to the 2001 ABS Census data for SA to provide estimates that were representative of the region's population. The comparison for age and sex using the ABS data used both weighted and un-weighted data. Significance was tested using SPSS V12.0 and EpiInfo Version 6 X
2 tests with a 0.05 level of significance. Adjusted standardized residuals were obtained using the methods of Haberman  and were used to test deviations from expected values separately in each cell. Bonferroni corrections were applied for multiple testing.