Toxicant- and response-specific comparisons of statistical methods for estimating effective concentrations

Standard U.S. Environmental Protection Agency (U.S. EPA) statistical analys es of whole effluent toxicity tests involve the estimation of the concentra tion associated with a specified level of inhibition relative to control re sponses. Current U.S. EPA estimation methods (linear interpolation or probi t-based methods) are compared to a recently developed parametric regression -based estimator, the relative inhibition concentration estimator RIp. The RIp estimation technique. with level of inhibition p = 25%, is applied to a series of chronic toxicity test data from a U.S. EPA Region 9 database of reference toxicity tests. Tests on marine species are conducted with one re ference toxicant, while the freshwater tests are conducted with several ref erence toxicants. While the U.S. EPA estimators and the RIP estimator are h ighly correlated for red abalone larval shell development, the degree of co rrelation for fathead minnow responses varies with reference toxicant teste d. The strength of the relationship between the RIP and the standard U.S. E PA estimators varies as a function of the reference toxicant. Correlations range between 0.67 and 0.99. For all biological responses included in this evaluation (fathead minnow growth or survival and red abalone larval develo pment), experiments occurred where the RIp is estimable, while the standard U.S. EPA estimators are not. Nonestimability of the standard EPA methods a ppears to be related, in part, to the failure of models to account for enha nced responses, such as a hormesis effect prior to toxicity being manifest. The ability to account for such enhanced responses is a strength of the RI P method. Finally, a variance component analysis suggests that lab-to-lab v ariability is relatively low for the red abalone but relatively high for th e fathead minnow.