Over the last few years, the educational interest of soft skills has steadily increased. Nonetheless, there is no considerable literature concerning the impact of followed courses on them. This simulation explores the modelling of the effects of courses over the students' soft skill proficiency, considering interaction effects and confounding variables through nine different scenarios. Moreover, the study compares the model with and without propensity scores as predictors, which are proposed as alternatives to handle the selection bias issue of the students' course choices. The simulation results show that in general models without propensity scores produce less bias, nonetheless the higher the effect of interactions between courses the better propensity scores become in reducing the bias.