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Methods

Introduction

In 2007, the NSW Department of Health, in conjunction with the 8 area health services, completed the sixth year of the New South Wales Population Health Survey, an ongoing survey of the health of people of New South Wales using computer assisted telephone interviewing (CATI). The main aims of the survey are to provide detailed information on the health of the people of New South Wales, and to support the planning, implementation, and evaluation of health services and programs in New South Wales.

Prior to the introduction of the continuous survey in 2002, the Centre for Epidemiology and Research conducted adult health surveys in 1997 and 1998, an older people's health survey in 1999, and a child health survey in 2001. The reporting plan for the continuous survey includes an annual report on adult health for the whole state and annual reports on adult health for selected indicators by area health service.

This section describes the methods used for the 2007 Report on Adult Health from the New South Wales Population Health Survey, which reports the health of residents aged 16 years and over.

New South Wales Population Health Survey

Survey instrument

The survey instrument for the New South Wales Population Health Survey was developed by the Health Survey Program in consultation with key stakeholders, area health services, other government departments, and a range of experts.

The survey instrument included: questions used in previous surveys, new questions developed specifically for 2007, and questions developed specifically for some area health services. All questions not previously used were submitted to the NSW Health Population and Health Services Research Ethics Committee for approval prior to use. New questions were also field tested prior to inclusion in the survey. The survey instrument was translated into 5 languages: Arabic, Chinese, Greek, Italian and Vietnamese.

Survey sample

In 2007, the target population for the New South Wales Population Health Survey was all residents living in households with private telephones. The target sample comprised approximately 1,500 people in each of the 8 area health services (a total sample of 12,000).

The sampling frame was developed as follows. Records from the Australia on Disk electronic white pages (phone book) were geo-coded using MapInfo mapping software.[1,2] The geo-coded telephone numbers were assigned to statistical local areas and area health services. The proportion of numbers for each telephone prefix by area health service was calculated. All prefixes were expanded with suffixes ranging from 0000 to 9999. The resulting list was then matched back to the electronic phone book. All numbers that matched numbers in the electronic phone book were flagged and the number was assigned to the relevant geo-coded area health service. Unlisted numbers were assigned to the area health service containing the greatest proportion of numbers with that prefix. Numbers were then filtered to eliminate contiguous unused blocks of greater than 10 numbers. The remaining numbers were then checked against the business numbers in the electronic phone book to eliminate business numbers. Finally, numbers were randomly sorted.

Households were contacted using random digit dialling. One person from the household was randomly selected for inclusion in the survey.

Interviews

In 2007, interviews were carried out continuously between February and December. Selected households with addresses in the electronic phone book were sent a letter describing the aims and methods of the survey 2 weeks prior to initial attempts at telephone contact. An 1800 freecall contact number was provided for potential respondents to verify the authenticity of the survey and to ask any questions regarding the survey. Trained interviewers at the Health Survey Program CATI facility carried out interviews. Up to 7 calls were made to establish initial contact with a household, and 5 calls were made in order to contact a selected respondent.

Call outcomes and response rates

In total, 16,046 interviews were conducted, with at least 1,550 interviews in each area health service and 13,178 with adults aged 16 years or over. The overall response rate was 63.6 per cent (completed interviews divided by completed interviews and refusals).

Data analysis

For analysis, the survey sample was weighted to adjust for differences in the probabilities of selection among subjects. These differences were due to the varying number of people living in each household, the number of residential telephone connections for the household, and the varying sampling fraction in each health area.

Post-stratification weights were used to reduce the effect of differing non-response rates among males and females and different age groups on the survey estimates. These weights were adjusted for differences between the age and sex structure of the survey sample and the Australian Bureau of Statistics 2007 mid-year population estimates (excluding residents of institutions) for each area health service. Further information on the weighting process is provided elsewhere.[3]

Call and interview data were manipulated and analysed using SAS version 8.02.[4] The SURVEYMEANS procedure in SAS was used to analyse the data and calculate point estimates and 95 per cent confidence intervals for the estimates. The SURVEYMEANS procedure calculates standard errors adjusted for the design effect factor or DEFF (the variance for a non-random sample divided by the variance for a simple random sample). It uses the Taylor expansion method to estimate sampling errors of estimators based on the stratified random sample.[4]

The 95 per cent confidence interval provides a range of values that should contain the actual value 95 per cent of the time. In general, a wider confidence interval reflects less certainty in the estimate for that indicator. The width of the confidence interval relates to the differing sample size for each indicator. A wider confidence interval reflects less certainty in the estimate. If confidence intervals do not overlap then the observed estimates are significantly different. If confidence intervals overlap slightly the observed estimates may be significantly different but further testing needs to be done to establish that significance.[4] For a pairwise comparison of subgroup estimates, the p-value for a two-tailed test was calculated using the normal distribution probability function PROBNORM in SAS, assuming approximate normal distribution of each individual subgroup estimates with the estimated standard errors, and approximate normal distribution for the estimated difference.

The K10 measure of psychological distress

In 2007, the K10 scale was included in the New South Wales Population Health Survey as a measure of psychological distress.[5,6] The K10 is a 10-item questionnaire intended to yield a global measure of psychological distress. It includes questions about the level of anxiety and depressive symptoms in the most recent 4-week period. For each question, there is a 5-level response scale based on the amount of time (from none of the time through to all the time) during a 4-week period that the person experienced the particular problem.

When scoring responses to the questionnaire, between 1 and 5 points were assigned to each symptom with a value of 1 indicating that the person experiences the problem none of the time and 5 indicating all of the time. It follows that the total K10 score for each person ranges from 10 points (that is, all responses are none of the time) through to 50 (all responses are all of the time.[7]

The K10 scores calculated for the New South Wales Population Health Survey are a combination of actual and imputed scores. Where a respondent answered all 10 questions, the K10 score was simply the sum of the individual scores for each question. Where the respondent answered 9 questions, the score for the missing question was imputed as the mean score of the 9 answered questions.

Indices of geographic remoteness and socioeconomic disadvantage: ARIA and SEIFA

The Accessibility-Remoteness Index of Australia Plus (ARIA+) is the standard Australian Bureau of Statistics (ABS) endorsed measure of remoteness.[8] It is derived using the road distances from populated localities to the nearest service centres across Australia. For each locality, the accessibility to services is expressed as a continuous measure from 0 (high accessibility) to 15 (high remoteness) and grouped into 5 categories: major cities, inner regional, outer regional, remote, and very remote. Because of small numbers in the remote and very remote categories, these categories have been combined in the analysis.

The Socio-Economic Indexes for Areas (SEIFA) describe the socioeconomic aspects of geographical areas in Australia, using a number of underlying variables such as family and household characteristics, personal educational qualifications, and occupation.[9] The SEIFA index used to provide breakdowns of the New South Wales Population Health Survey data in 2007 is the Index of Relative Socio-Economic Disadvantage. This index is calculated on attributes such as low income and educational attainment, high unemployment, and people working in unskilled occupations. The SEIFA index values are grouped into 5 quintiles, with quintile 1 being the least disadvantaged and quintile 5 being the most disadvantaged.

Both the ARIA+ and SEIFA indexes were assigned to the results of the New South Wales Population Health Survey in 2007 based on respondents' postcode of residence. Rates for each SEIFA quintile were calculated for several health indicators included in this report to enable socioeconomic comparisons.

Definition of urban and rural

In this report, the term urban means the respondent lived in 1 of the 4 area health services designated as metropolitan: Northern Sydney & Central Coast, South Eastern Sydney and Illawarra, Sydney South West, and Sydney West. The term rural means the respondent lived in 1 of the 4 area health services designated as rural: Greater Southern, Greater Western, Hunter & New England, and North Coast.

Nutrition

Adequate fruit and vegetable consumption is defined in the Australian Guide to Healthy Eating, the Dietary Guidelines for Children and Adolescents, and the Dietary Guidelines for Australian Adults. Recommended amounts vary with age. For adolescents up to age 18 years, at least 3 serves of fruit and at least 4 serves of vegetables are recommended. From 18 years, it is recommended that adults eat at least 4-5 serves of vegetables per day, depending on age, and at least 2 serves of fruit per day. The 'Go for 2 & 5' fruit and vegetable campaign provides a simplified message that has been used as the basis for comparison in this survey.[10-12]

The Dietary Guidelines for Australian Adults state that people should limit the consumption of saturated fats, and choose foods that are low in salt, without making any specific recommendations.[11] However the National Food and Nutrition Monitoring and Surveillance Project recommends monitoring the percentage of the population that rarely or never eats fried potatoes, rarely or never eats salty snacks, and consumes meat products less than 3 times a week.[13]

The Dietary Guidelines for Australians recommends serves of cereals (including breads, rice, pasta, and noodles) based on age, sex, and individual circumstances.[14] For ease of respondent recall, the National Food and Nutrition Monitoring and Surveillance Project recommends breaking the cereals category into sub-categories: that is, collecting the frequency of consuming breads, cooked cereals, and breakfast cereals.[13] Thus the National Food and Nutrition Monitoring and Surveillance Project recommends comparing those who consume bread daily or more; rice, pasta, noodles, or other cooked cereals daily or more; and breakfast cereals 2 or more times a week, with those who do not.

The New South Wales Population Health Survey includes a dietary questionnaire on usual consumption of fruit and vegetables, breads and cereals (including pasta, rice and noodles), type of milk consumed (including low fat, reduced fat, and skim milk), selected foods high in fats (fried potatoes, potato crisps and salty snacks, and processed meats), red meat (excluding pork or ham), soft drinks, fast foods, knowledge of recommended servings of fruit and vegetables, and food insecurity.[13] Several of these questions were validated using the 1995 National Nutrition Survey and the Tasmanian Dietary Key Indicators Study. The validated questions were found to be reliable for relative ranking of intake between respondents but not for measuring a respondent's number of serves; however, they are still useful for ongoing comparitive monitoring.[15]

References

  1. Australia on Disk [software]. Sydney: Australia on Disk, 2000.
  2. MapInfo [software]. Troy, NY: MapInfo Corporation, 1997.
  3. Williamson M, Baker D, Jorm L. The NSW Health Survey Program: Overview and methods 1996-2000. N S W Public Health Bull 2001; 12(S-2). Available online at www.health.nsw.gov.au/publichealth/surveys/index.asp (accessed 5 September 2008).
  4. SAS Institute. The SAS System for Windows version 8.2. Cary, NC: SAS Institute Inc., 2001.
  5. Kessler R, Mroczec D. An update of the development of mental health screening scales for the US National Health Interview Survey. Ann Arbor MI: Survey Research Centre of the Institute for Social Research, University of Michigan, Memo dated December 22, 1992.
  6. Kessler R, Mroczec D. Final versions of our Non-Specific Psychological Distress Scale. Ann Arbor MI: Survey Research Centre of the Institute for Social Research, University of Michigan, Memo dated March 10, 1994.
  7. Australian Bureau of Statistics. Information paper: Use of the Kessler Psychological Distress Scale in ABS Health Surveys. Catalogue no. 4187.0.55.001. Canberra: ABS, 2003.
  8. Australian Bureau of Statistics. ASGC Remoteness Classification: Purpose and Use. Census Paper No. 03/01. Commonwealth of Australia, 2003.
  9. Australian Bureau of Statistics. 1996 Census of Population and Housing: Socio-Economic Indexes for Areas, Information Paper, Catalogue no. 2039.0. Canberra: ABS, 1998.
  10. National Health and Medical Research Council. Australian Guide to Healthy Eating. Canberra: NHMRC, 2003. Available online at www.health.gov.au (accessed 5 September 2008).
  11. National Health and Medical Research Council. Dietary Guidelines for Australian Adults. Canberra: NHMRC, 2003. Available online at www.nhmrc.gov.au (accessed 5 September 2008).
  12. 'Go for 2 & 5' fruit and vegetable campaign website at www.healthyactive.gov.au (accessed 5 September 2008).
  13. Marks G, Webb K, Rutishauser I, and Riley M for the National Food and Nutrition Monitoring and Surveillance Project. Monitoring food habits in the Australian population using short questions. Canberra: Australian Food and Nutrition Monitoring Unit and Commonwealth Department of Health and Aged Care, 2001. Available online at www.health.gov.au/internet/main/publishing.nsf/Content/health-pubhlth-strateg-food-pdf-foodhabits-cnt.htm (accessed 5 September 2008).
  14. National Health and Medical Research Council. Food for health: Dietary Guidelines for Australians. Canberra: Australian Government Department of Health and Ageing, 2005. Available online at www.nhmrc.gov.au/publications/synopses/dietsyn.htm (accessed 5 September 2008).
  15. Rutishauser IHE, Webb K, Abraham B, Allsop R. Evaluation of short dietary questions from the 1995 National Nutrition Survey. Canberra: Australian Food and Nutrition Monitoring Unit, Commonwealth Department of Health and Aged Care, 2001. Available online at www.health.gov.au/internet/main/Publishing.nsf/Content/health-pubhlth-strateg-food-pdf-evaluation-cnt.htm (accessed 5 September 2008).

Graphs

Source: New South Wales Population Health Survey 2007 (HOIST). Centre for Epidemiology and Research, NSW Department of Health.
Print version: Although this page can be printed directly from your web browser, a higher quality version is available as a PDF file that can be printed or viewed on screen.
Produced by: Centre for Epidemiology and Research, Population Health Division, NSW Department of Health.
Last updated on: 1 October 2008

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