The Minimum Significant Difference at the NOEC calculated with a non-parametric test.
Nelly van der Hoeven
2332 AA Leiden
For many ecotoxicity tests, the statistic summarising the effect size is the so-called No Observed Effect Concentration (NOEC) (or ~Dose (NOED)), which is the highest test concentration (or dose) without a statistically significant result. The NOEC has many drawbacks (, ), one of these being that the NOEC will be larger if the experiment is performed with less replicates or under less controlled conditions with a larger within-replicate variance. Therefore, when the NOEC is used, it should be accompanied by some measure that indicates how much difference in response should be observed minimally for a response to be found significantly different from the response observed in the control treatment. (, ). The Minimum Significant Difference (MSD) is such a measure.
Many guidelines prescribe that the MSD or the confidence interval for the deviation from the response observed in the control treatment must be given (, ). In standard statistical software, the MSD can only be calculated for parametric statistical tests based on the normal probability distribution (e.g. Student's t-test and Dunnett's test). In the poster, a method will be presented to calculate the MSD if the non-parametric Wilcoxon rank-sum test is used to compare the observed responses at each of the concentrations with the responses observed in the control treatment.
 Chapman, P.F. et al., 1995. Asking the right questions: ecotoxicology and statistics. In: Report workshop Royal Holloway Un. London, UK
 van der Hoeven, N., 1997. How to measure no effect, part III: Statistical aspects of NOEC and ECx estimates. Environmetrics 8: 255-261.
 USEPA, 2000. EPA/833/R-00/003.
 OECD, 2006. ENV/JM/MONO(2006)18
In: Proceedings of the 30th Anniversery Meeting of the Netherlands Society of Toxicology, june 2009. p. 122