Investigating changes over time in socioeconomic gaps in cancer survival: using differences in relative survival versus differences in excess mortality rates can give different answers

Trends in ethnic and socioeconomic inequalities in cancer survival, New Zealand, 1991-2004

Improvements in cancer survival may be distributed inequitably throughout populations and across time. We assessed trends in cancer survival inequalities in New Zealand by ethnic and income group. 126,477 people diagnosed with cancer between 1991 and 2004, followed-up to 2006, were included. First, inequalities pooled over time were measured with excess mortality rate ratios (EMRRs). Second, interpretation of changes in inequalities over time can differ depending on whether one uses EMRRs, excess mortality rate differences (EMRD) or absolute differences in relative survival risks (RSRD); we estimated all three by cancer-site and (for EMRRs only) pooled across all sites. We found that pooled over time and all sites, Māori had an EMRR of 1.29 (95% CI, 1.24-1.34) compared to non-Māori. The low compared to high-income EMRR was 1.12 (95% CI, 1.09-1.15). Pooled over cancers, there was no change in the ethnic EMRR over time but the income EMRR increased by 9% per decade (1-17%). Changes over time in site-specific inequalities were imprecisely measured, but the direction of change was usually consistent across EMRRs, EMRDs and RSRDs. There were persistent ethnic inequalities in cancer survival over time, and slower improvements for low-income people.