Q1: Is class size positively related to thinking skills already in thebeginning of the fourth grade due to the use of class size regulation as ameans of support?H1. The positive correlation of class size can already be seen in thefourth grade as students receiving support (tier 2 and tier 3) study inclasses which are on average smaller.Q2: Is class size a predictor of later performance or does it simplyreflect initial differences between the student populations of theclasses?H2. The gap between smaller and larger classes increases over time.One important reason for this is that students' initial performance levelis higher in larger classes, which also have less students with supportneeds. At an individual level, higher initial level often predicts betterlater acquisition of the same skills (cf., Bast & Reitsma, 1997) andstudents with support needs tend to slowly fall behind (Vainikainen,2014).