One of the major challenges of the German vocational education and training (VET) system is the integration of adolescents with unfavorable learning prerequisites, i.e., individuals with no or lower school-leaving certificates or low levels of basic competencies (e.g., reading and mathematics). This paper examines the extent to which trainees’ learning prerequisites affect the development of professional knowledge during VET. In addition, the analysis focuses on the role of learning conditions during VET in reducing inequalities that stem from differences in trainees’ learning prerequisites. This paper implements a longitudinal design to examine within-person effects regarding the professional knowledge of N = 401 prospective chefs in a German dual VET program over the first two years of VET. Based on a domain-specific knowledge test, weighted likelihood estimates (WLE) are calculated as person parameter estimates for trainees’ professional knowledge at the beginning (t<inf>1</inf>) and the second year of VET (t<inf>3</inf>). Following the idea of value-added modeling, professional knowledge at t<inf>3</inf> serves as the dependent variable in a structural equation model. Trainees’ professional knowledge level at t<inf>3</inf> is modeled as a function of the initial professional knowledge level (t<inf>1</inf>) and trainees’ learning prerequisites when entering the VET program (educational background and basic competencies) as well as learning conditions (work assignments, social interactions, and feedback) during the first year of VET (t<inf>2</inf>). Moderation effects additionally examine whether trainees with less favorable learning prerequisites benefit more from high-quality learning conditions. The study yields two primary findings. First, the association between trainees’ learning prerequisites and their professional knowledge decreases over the course of VET. Controlling for initial levels of professional knowledge, only trainees’ reading skills are found to be significantly directly associated with professional knowledge at t<inf>3</inf>, i.e. the second year of VET (β = 0.209, p <.01), while mathematical skills are shown to be indirectly related to professional knowledge at t<inf>3</inf> (via initial knowledge levels). Similarly, the association between trainees’ educational background and professional knowledge at t<inf>3</inf> is mediated via basic competencies. Hence, when controlling for initial professional knowledge levels, the role of trainees’ learning prerequisites in subsequent knowledge acquisition during VET is rather limited. Second, the results underline the importance of learning conditions during in-company training periods in VET – especially for trainees with unfavorable learning prerequisites. Controlling for trainees’ learning prerequisites and initial professional knowledge levels, the variety of work assignments during in-company training is significantly positively associated with professional knowledge gains (β = 0.156, p <.05). In addition, the moderation analyses show that trainees’ learning prerequisites (educational background, reading comprehension skills, and mathematical knowledge) moderate the association between all three learning conditions examined and professional knowledge acquisition during VET. This means that trainees with less favorable learning prerequisites benefit more strongly from high-quality learning conditions. Implications for the selection of trainees as well as the design of VET programs are discussed. © The Author(s) 2025.
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