Ecotoxicological studies performed for the authorization of plant protection products (PPP) usually result in the reporting of endpoint values in terms of effect concentration (EC) affecting a percentage x of test organisms or where a x percentage of an effect is observed (EC x ). The new Regulation (EC) No. 1107/2009 for the authorization of PPPs and the related data requirements provide that ecotoxicological endpoint data from chronic or long-term studies submitted by the Applicant are reported as EC 10 or EC 20 values together with the NOEC. NOEC values have been criticized since their values strongly depends on the experimental study design, whereas EC x values take into account the whole concentration-response curve and are therefore considered more appropriate. The aim of the project is to investigate the comparability of the EC x approach to the current NOEC approach on a larger data sets in view of the new Regulation requirements. Ecotoxicological data gathered from 70 active substances' approval dossiers were collected and stored into a MS Access database. All the extracted ecotoxicological data were analyzed in order to derive NOEC and calculate EC 10 , EC 20 , EC 50 with confidence intervals, using statistical models from the exponential and Hill families for continuous data, and logistic, log-logistic and complementary log-log models for quantal data. The optimal model was selected based on likelihood ratio tests and the Akaike Information Criterion. EC x /NOEC ratio distributions were calculated considering the whole set of data and model outputs; data were grouped in different categories to remark any differences in the EC x /NOEC ratio distributions.
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