Extrapolating ecotoxicological measures from small data sets.

Risk screening is commonly conducted using multiple-species ecotoxicological measures such as the HC5, the hazardous concentration at which 5% of species in a specified (eco-)system are assumed to be stressed. This article demonstrates that the estimate of HC5 will not vary significantly among commonly adopted parametric models of species sensitivity distributions. Uncertainty is highly dependent on the number of species tested (sample size) and the relevance of the measurement to the assessment endpoint (e.g., acute measures for assessing chronic endpoints). This article cross-compares estimates of these uncertainties using different empirical and theoretical methods to propose sample to population extrapolation factors. Some theoretical parametric methods for estimating the confidence intervals on the HC(5) can result in large overconservatism, particularly if positive bias reduces uncertainty. The 95th percentile confidence interval on the HC5 estimate given only three chronic test results varies from 5 to 8 x 10(8), depending on the estimation method adopted.