The evaluation of uncertainly is a requirement for all laboratories accredited according to the ISO/IEC 17025 standard. Nevertheless, there are no set rules for the evaluation of uncertainly in the testing of environmental samples. The sample matrix is one of the main sources contributing to the uncertainly of results obtained in the testing of environmental samples. The sample matrix is very difficult to quantify as a component of uncertainly, as it requires a large amount of experimental data. Efforts were made to estimate the sample matrix’s effect and expanded uncertainly for two environmental test methods. Statistical methods were used in the treatment of experimental data received from method validation, quality assurance and quality control measures. Results showed that uncertainly evaluated using a mathematical model incorporating all individual uncertainly components is not significantly different from the uncertainly calculated directly from experimental data (X-control charts) characterizing overall method performance.