An experimental exercise on analytical proficiency studies in collaborative trials is proposed. This practical provides students in advanced undergraduate courses in chemistry, pharmacy, and biochemistry, with the opportunity to improve their quality assurance skills. It involves an environmental analysis, determining the concentration of a contaminant (hexavalent chromium) in a synthetic reference sample simulating wastewater. A variety of analytical measurement techniques were compared: molecular absorption spectroscopy, flame atomic absorption spectroscopy, and differential pulse polarography. Furthermore, several complementary techniques for sample preparation such as liquid-liquid extraction and co-precipitation were also involved. The students acted as individual laboratories in a round-robin test, and the analytical results were evaluated in terms of precision and accuracy using a range of statistical tools such as visual display methods. Several causes of systematic error and strategies to improve the reliability of measurements were discussed.