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 B.
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. 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. Those over working age or who are full-time students in each wave are excluded from the analysis.
As with all surveys, not all those approached to take part 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. Statistics New Zealand provides a standard longitudinal weight that 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.
For these reasons the results in this paper reflect the SoFIE population, who are likely to be somewhat healthier than both the population it aims to represent and the New Zealand population more generally. More specifically, those with the most severe health conditions considered may die or be institutionalised, and so are not covered by the survey results used in this analysis. Therefore, the impact of the health conditions considered in this study on labour force participation may 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 B.
