There are lots of different ways of measuring income distribution. In The Spirit Level we averaged the figures published in the UN Human Development Reports for the years 2003 to 2006. The UN figures for the rich countries are basically the same as the ones published by the World Bank and came originally from the Luxembourg Income Study (LIS) which was set up to help produce internationally comparable figures of income inequality.

Whether you use the OECD inequality figures for the mid 2000s or those from the UN makes rather little difference to the results. The OECD Gini coefficients of inequality are strongly correlated with our Index of Health and Social Problems (r=0.7, p<0.001) – only a little weaker than the relation illustrated in Figure 2.2 in The Spirit Level. The relationship is slightly weaker because Japan becomes an outlier on the OECD figures. Although we have a rule that we take the data exactly as given in our sources without picking and choosing, if you preferred to take Japan out of the analysis – on the grounds that there is so much difference in its inequality figures from these two sources – the relationship using the OECD figures becomes stronger still.

We do not know why the Gini coefficients published by the OECD and the UN are so different, they differ substantially for several countries, including Japan, the Netherlands, Belgium, Switzerland and France. We know of no good reason to prefer one data series over another.

However, we have checked the robustness of the analyses in The Spirit Level, using the OECD Gini coefficient for the mid-2000s.