642 N majority N minority Total N Table 1 Distribution of students over classes varying in the proportion minority students Proportion \0.10 2,703 109 2,812 Proportion 0.10–0.30 1,427 278 1,705 may have skipped a grade in primary school or repeated a year in either primary school or high school, the sample includes also 11- and 18-year-olds. It should be noted though that the number of students in these age categories is neglectable. Education level was assessed by asking the pupils about the type of school they were enrolled in. Considering the Dutch educational system the possible answers ranged from lower general secondary education (1) to the highest level of secondary school (7). A higher score indicates a higher level of education. Class size refers to the number of students in class. Family wealth is assessed with the Family Afﬂuence Scale (FAS) that is speciﬁcally developed for use in research on adolescents, as it is difﬁcult to assess parental occupation or family income through the eyes of children. The FAS consists of four questions reporting on the presence of material goods in the family: number of cars, pupil having a bedroom of his/her own, number of computers in the home and number of times per year the family goes on holiday. The FAS has been validated in earlier research [8]. Missing values To minimise non-response on problem scores of the YSR, regression imputation was used to estimate missing values. This was done only for pupils who had 8 or less missing on all 101 items of the YSR, as this is considered a maximum number of missing items for computation of a total prob- lem score [33]. This resulted in a ﬁnal sample of 4,563 respondents (Nmajority = 3,790, Nminority = 773). Data analysis By using the SPSS Mixed modelling procedure, we examined two types of effects. First, the random effect of minority status—that is, does the relation between minority status and problem behaviour differ between classes. Sec- ond, we examined class and individual ﬁxed effects—that is, does the proportion of minority students in classes make these students differ in their level of internalising and externalising problem behaviour, when controlling for individual-level factors. Hence, a multilevel regression analysis was carried out to test for cross-level interactions between ethnic minority status at the individual level and the proportion of ethnic minority students at the class level. 123 Soc Psychiat Epidemiol (2010) 45:639–646 Proportion 0.50–0.70 80 98 178 Proportion 0.70–0.90 36 136 172 Proportion [0.90 3 121 124 Proportion 0.30–0.50 298 189 487 Results Since the data ﬁle has a hierarchical structure, preliminary to our analysis it was examined whether there is any var- iation at the class-level. An intercept-only model with only the student level was compared to an intercept-only model with two levels, including the class level. For both inter- nalising and externalising problem behaviour, the two-level model ﬁtted the data signiﬁcantly better (internalising v2(1) = 42.26, P = 0.000, externalising v2(1) = 93.14, P = 0.000), indicating that next to variation at the indi- vidual level, also class level variation exists. The intra- class-correlation of 0.039 for internalising problem behaviour is rather low, and somewhat higher for exter- nalising problem behaviour (0.061). The intraclass corre- lation refers to the expected correlation between two randomly chosen students within the same class [13]. The multilevel regression models of internalising and exter- nalising problem behaviour without covariates both showed a signiﬁcant random effect of ethnic minority status (internalising v2(1) = 4.324, P = 0.04, externalising v2(1) = 9.676, P = 0.002), indicating that minority stu- dents differ between classes with respect to their levels of internalising and externalising problem behaviour. Table 2 shows the effects of the explanatory variables on the two dimensions of problem behaviour at individual and class level. No difference is found between the Dutch majority and ethnic minority students with respect to their level of internalising problem behaviour. However, we do ﬁnd a signiﬁcant positive relation between ethnic minority status and externalising problem behaviour. Hence, ethnic minority students on average report higher levels of externalising problem behaviour, even when we take into account other factors such as age, sex, educational level and class size. In the next step, cross-level interactions between minority status and the proportion of minority students in class were tested. With respect to internalising problem behaviour, we did not ﬁnd evidence for a moderating effect of ethnic composition. The earlier reported random effect of minority status could not be attributed to differences in the proportion of minority students in class (v2(1) = 0.832, P = 0.181). However, with respect to the level of exter- nalising problem behaviour, we did ﬁnd a signiﬁcant neg- ative interaction between ethnic minority status at the