It is a well-accepted finding that the socioeconomic background of students is closely related to student learning and learning outcomes (UNICEF 2002). Looking at the conceptual and methodological underpinnings it becomes evident that over the last decade’s international educational research has used a variety of proxies to control for diverse educational opportunities. Thrupp et al. (2002) question the validity of the most common measures used. This paper addresses the issue by describing a method of gathering theoretically based information on the social background of students at the individual level. The choice of indicators used is based on the work of Bourdieu (1973, 1990) and Coleman (1996) (cp. Bonsen et al. 2007, 2010). By making use of IRT-technology, we show how to construct an index, that admits quantification and appraisals of the social and familial capabilities of students from different schools on a latent continuum from ‘disadvantaged‘ to ‘privileged‘ status of schools (cp. Bos et al. 2006). On the basis of the data resulting from the ADDITION-Study the cross-cultural validity of the index will be investigated. It illustrated how the social index can be used to describe intake characteristics and for ‘fair comparisons‘ of the effectiveness of different primary schools.