Income is an important driver of health inequalities in New Zealand (for example see the New Zealand Census Mortality Study) and it is important to take into account when studying health outcomes in the Integrated Data Infrastructure (IDI). There are many ways of examining income. We discuss the two main sources of income information available to non-government IDI users. The first is summary income information from the Inland Revenue Department (IRD) and the second is income reported in the 2013 census.

IRD income

The strength of this IRD personal income data is that it provides an objective and longitudinal measure of a person’s income over time, and it can be cut in many different ways, whilst also accounting for time outside the country, eg, income in past year, average income over past five years, changes in income etc. However, researchers outside of government do not have access to business data including self-employed income (due to the IR Tax Act). For this reason, a more restricted aggregated set of personal income data has been created for external use. The Employer Monthly Schedule (EMS) table ‘ems_cal_yr_summary’ was produced in 2015 and is located in the IDI sandpit for the years 1999 to 30 Sep 2015 (currently).

The main caveats are that this table excludes information on investment income, self-employed income (including salaried self-employed income), income not taxed at source (eg the accommodation supplement, a non-taxable government transfer) and income earned and taxed overseas. Furthermore, data on some individuals may be repressed including sensitive occupations such as judges. The table includes information on benefits paid for MSD’s main benefits. These are all coded under the source code ‘BEN’ and currently include unemployment benefits, domestic purposes benefits, sickness benefits, invalid’s benefits, widow’s benefits and emergency benefits (but are not disaggregated by these benefit types). Pension information is under the income source code ‘PEN’. Other source codes include wages and salary (W&S), withholding payment (WHP), student allowance (STU), paid parental leave (PPL) and claimants compensation (CLM). In the Healthier Lives Earthquake Cardiovascular Disease project, we found that in 2010 income records were available from the IRD for approximately 90% of the population 25+ years old. With improved information on household composition it may be possible to use this longitudinal data to measure household income over time including how it may change from month to month.

Census income

The second main source of income information in the IDI is from the 2013 census. There are many ways of creating summary income variables depending on the research question. The Jensen Equivalised Income is one we have found useful for our particular research to account for the composition of the household (Ministry of Health, 2005). Jensen Equivalised income tends to separate lower incomes and takes account separately the number of adults and children in a household, whereas the Luxembourg Equivalised Income method creates a different income distribution and only uses the total number of people in the household. Jensen equivalised income is used routinely by Statistics New Zealand. This method is also used to create a variable used in the New Zealand Deprivation Index, and in the analyses in the New Zealand Census-Mortality and CancerTrends studies for equivalised household income quintiles (Ministry of Health, 2005). An example of code will be posted on this website soon – see code sharing. Finally, census income can also be analysed at a personal level. An example of code that extracts personal income is available from the Virtual Health Information Network catalyst projects (code here).

For more information on the IRD information in the IDI please see;

References

MINISTRY OF HEALTH 2005. Decades of Disparity II Socioeconomic mortality trends in New Zealand, 1981–1999. Public Health Intelligence Occasional Bulletin Number 25. Wellington: Ministry of Health. Accessible; http://www.otago.ac.nz/wellington/otago024500.pdf

By Andrea Teng, June Atkinson

Version: Original 3 April 2017 and updated 2 May 2017.