Sediments from 71 river sites in Northern Spain were tested using the oligochaete Tubifex tubifex (Annelida, Clitellata) chronic bioassay. 47 sediments were identified as reference primarily from macroinvertebrate community characteristics. The data for the toxicological endpoints were examined using non-metric MDS. Probability ellipses were constructed around the reference sites in multidimensional space to establish aclassification for assessing test-sediments into one of three categories (Non Toxic, Potentially Toxic, and Toxic). The construction of such probability ellipses sets the Type I errorrate. However, we also wished to include in the decision process for identifying passâ¿¿fail boundaries the degree of disturbance required to be detected, and the likelihood of being wrong in detecting that disturbance (i.e. the TypeIIerror). Setting the ellipse size to use based on Type I error does not include any consideration of the probability of TypeIIerror. To do this, the toxicological response observed in the referencesediments was manipulated by simulating different degrees of disturbance (simpactedsediments), and measuring the TypeIIerrorrate for each set of the simpactedsediments. From this procedure, the frequency at each probability ellipse of identifying impairment using sediments with known level of disturbance is quantified. Thirteen levels of disturbance and seven probability ellipses were tested. Based on the results the decision boundary for Non Toxic and Potentially Toxic was set at the 80% probability ellipse, and the boundary for Potentially Toxic and Toxic at the 95% probability ellipse. Using this approach, 9 test sediments were classified as Toxic, 2 as Potentially Toxic, and 13 as Non Toxic.