The measurement uncertainty is required for the objective and sound interpretation of the measurement result. However, the evaluation of the uncertainty of complex measurements in chemistry, such as for the analysis of trace levels of analytes in environmental or biological samples after complex sample preparation, is not trivial. Some examples of complex sample preparations are acid digestions of the sample for the elemental analysis or the solvent extraction of organic analytes. This communication presents a novel approach for the “subanalytical” evaluation of complex sample preparation that involves the isolation and the Monte Carlo Method simulation of the precision and trueness of the analytical steps. The simulation is based on observed experimental performance since the analytical efficiency frequently varies with uncontrolled characteristics of the analysed item.