In Germany the dual vocational education system plays a central role in preparing young people for work. In order to succeed on the labour-market they need competences that go way beyond knowledge and cognitive skills. A quarter of trainees however dropout of vocational education raising questions about their professional action competence. This paper discusses the issue as to what extent subjective ideas of trainees from different occupational fields about their competences compiled using a questionnaire for Social, Methodical, and Personal Competences (SMP-C) are reliable and valid regarding the social, methodical, and personal competence classes. This entails an examination as to whether the SMP-C competence assessment questionnaire features satisfactory psychometric characteristics, on how the formed ability concepts behave towards each other and other constructs, and which competence values can be diagnosed. In order to examine these questions 937 trainees were asked about their competences and tendencies to dropout of vocational training. The analysis attests the assessment procedure overall satisfactory to good reliability, which speaks in favour of surveying social, methodical, and personal ability concepts using the subjective ideas of trainees regarding the application of competences. Furthermore, it seems that a stronger manifestation of specific social, methodical, and personal competences reduces the tendency to dropout of vocational training. Hence it might be possible to prevent early dropout from vocational training through specific strengthening of these competences.