In most cases, the income surveys used for the OECD Income Distribution Database (IDD) are not designed to be representative at the regional level. The problem is particularly evident for city-regions. For example, the Spanish cities of Ceuta and Melilla are represented respectively by 113 and 114 households in the European Union Statistics on Income and Living Conditions (EU-SILC) 2011 sample, similarly, the regions of Bremen and Hamburg in Germany are represented by, respectively, 66 and 166 households in the German Socio-Economic Panel for 2011. These representativeness issues are discussed in detail in section 4 and in the annexes. Figure 1 shows, for each country, the ratios between the highest and the lowest value of GDP per capita, household market income and household disposable income data observed across regions. This evidence suggests large cross-country differences in the level of interregional disparities as measured by GDP and household income data.