One reason why the association is weak, even before accounting for the socio-economic profile of students and schools, is that the relation is curvilinear: students who had spent too little time (less than one year) in pre-primary education score lower in science than students who had not attended or who had spent more than one year (Table 11.6.52). This only covers expenditure on educational institutions. These resources are allocated throughout a student's educational, and countries spend different amounts per student. Caution is required in interpreting this indicator, as school systems are organised in many different ways across countries.