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The Cost of Ill Health WP 10/04

3  Data

3.1  Survey methodology

The Survey of Family Income and Employment (SoFIE) is the main data source analysed in this paper. SoFIE is a survey of a nationally representative sample of New Zealand permanent residents in private households. It is conducted by Statistics New Zealand. The core SoFIE survey modules include questions on demographics; dependent children; labour force involvement; education; family and income. All respondents in the original sample are followed over time, even if their household or family circumstances change, forming a longitudinal sample. The survey commenced in 2002 and will continue until 2010. When the present study was undertaken, there were three waves of data available for analysis (SoFIE Waves 1-3 Version 4). Further information on the survey methodology can be found in Appendix A.

3.2  Population and sample of interest

The analysis is based on those people who remain eligible and respond in Waves 1-3 who are aged 15 and over at the end of the reference period in Wave 1, as this is the group that were asked the health module in Wave 3. So they will be aged 17 and over in this analysis. The results are therefore representative of the usual adult resident population of New Zealand who lived in private dwellings on the main islands of New Zealand in 2002/03 and who remain alive and are non-institutionalised by 2004/05.

As with all surveys, not all those approached to take part in the survey agree to participate. In addition, those who initially respond may choose not to respond in subsequent waves of the survey (attrition). While the response rates are good compared with similar surveys, longitudinal response rates were lower for those of fair or poor health compared with those of better health.[6] Statistics New Zealand provides a longitudinal weight (standard longitudinal weight) which accounts for non-response and aligns the composition of the sample with that of the New Zealand population in October 2002 in terms of age, gender and Māori. However, the weights do not completely restore the distribution of people across the health states. The implication of this is that the SoFIE population is likely to be healthier than the population it represents and the New Zealand population more generally. More specifically, those with the most severe cases of the health conditions considered will die or may be institutionalised and therefore are not covered by the survey. Consequently the actual component costs of ill health that are estimated in this study are likely to be higher than the results based on SoFIE suggest. Further information on the limitations and strengths of SoFIE more generally can be found in Appendix A.

In Wave 3, together with the social and economic questions that are asked in every wave, the original sample members over 15 years old in that wave were asked a detailed set of health questions. At the end of the health module respondents were asked to give permission for their data to be linked to information on hospitalisations, cancer registrations and future death registrations held by New Zealand Health Information Service (NZHIS). The Statistics New Zealand weight does not allow for non-consent. While there was no difference in consent rates by self-rated health, those with a higher number of chronic diseases were significantly more likely to agree to the linkage. All analysis in this paper is based on the linked sample with adjusted longitudinal weights used to realign the sample with the population (adjusted longitudinal weight) as opposed to the weights provided by Statistics New Zealand (standard longitudinal weights).[7][8]

3.3  Variables considered

The core SoFIE modules include questions on demographics; dependent children; labour force involvement; education; family and income.[9] The main SoFIE health variables used in this analysis are self-rated health, whether activity has been stopped by illness and how often a person's activities have been reduced by their physical or mental health. As well as the detailed information asked in the SoFIE survey, for those matched, consenting SoFIE respondents, inpatient hospital treatment information back to 1990 is available. This information includes the age at discharge; gender; International Statistical Classification of Diseases and Related Health Problems code (ICD code); Diagnosis-related Group (DRG); date of admission; date of discharge; length of stay; and facility type for each hospital episode. The hospital inpatient information does not include records for appointments of less than three hours (ie, outpatient appointments) or information on appointments with primary health care providers such as GPs. It will also not include information on private hospital treatments; treatments at these facilities are only included if the treatment is publicly funded. These health variables, along with the non-health explanatory variables used in some of the models, are defined in Appendix Tables B1, B2 and B3 of Appendix B.


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