Bias assessment (systematic errors) is fundamental in industry and service laboratories, where reliable results must be obtained to give correct answers to specific problems. Therefore, knowledge and practice in quality methodologies is of fundamental importance for students. Unfortunately, laboratory lessons often focus on connecting theory and practice without developing the quality assurance skills that employers consider to be useful. Here, an external quality methodology based on proficiency testing schemes has been developed to introduce students to the methodologies and skills that are required to detect and find the source of biased results in the laboratory. It has been found that a large percentage of results obtained during laboratory lessons are in fact biased. The new methodology obliges students to practise how to detect biased results and, specially, the critical thinking skills that are needed to reveal the sources of bias in the laboratory. In the case study evaluated here, the application of the proposed methodology allowed students to detect the source of their biased result in 95% of the cases. Errors found were mainly due to personnel errors, as a consequence of deficits in basic hands-on skills and calculation errors.