But even after accounting for the socioeconomic status and demographic background of students and schools and various other school characteristics, in the Czech Republic, Slovenia and Switzerland schools whose principals reported that teacher shortages hinder learning tend to show lower average performance (Table IV.1.12c). On average across OECD countries, almost half of the performance differences between schools are accounted for jointly by school resources and students' and schools' socioeconomic status and demographic profile (Table IV.1.8a).16 This suggests that much of the impact of socio-economic status on performance is mediated by the resources invested in schools. As shown in Figure 1V.1.11, even after accounting for per capita GDP, 30% of the variation in mathematics performance across OECD countries can be explained by the level of similarities in principals' report on school s' educational resources between socio-economically advantaged and disadvantaged schools.