According to the MELS (2008, 2010), 30% of students enrolled in a vocational training program (FP) in Quebec drop out or fail in their program of study. This is confirmed by principals and teachers of vocational training, especially when the program includes mathematical content, such as construction. This study therefore aims to better define the nature of difficulties in mathematics for students in vocational training. Using rasch modeling, the study prioritizes diagnostic purposes in mathematics as perceived by teachers and school departments who completed a questionnaire. The results highlight the difficulties of students in several areas of mathematics and diagnostic purposes in particular with respect to logical reasoning, calculation techniques, approaches requiring multiple steps and understanding of the problem statement.