Sediment Quality in Puget Sound Year 2 - Center for Coastal ...

arcsine square root transformed data with the aid of SAS (SAS, 1989). The trimmed Spearman-
Karber method (Hamilton et al., 1977) with Abbott's correction (Morgan, 1992) was used to
calculate EC50 (50% effective concentration) values for dilution series tests. Prior to statistical
analyses, the transformed data sets were screened for outliers (Moser and Stevens, 1992).
Outliers were detected by comparing the studentized residuals to a critical value from a t-
distribution chosen using a Bonferroni-type adjustment. The adjustment is based on the number
of observations (n) so that the overall probability of a type 1 error is at most 5%. The critical
value (CV) is given by the following equation: cv= t(dfprror, .051[2 x n]). After omitting outliers
but prior to further analyses, the transformed data sets were tested for normality and for
homogeneity of variance using SASILAB Software (SAS, 1992). Statistical comparisons were
made with mean results from the Redfish Bay controls. Reference toxicant concentration results
were compared to filtered seawater controls and each other using both Dunnett's t-test and
Duncan's multiple range test to determine lowest observable effects concentrations (LOECs) and
no observable effects concentrations (NOECs).
In addition to the Dunnett's one-tailed t-tests, data from field-collected samples were treated with
an analysis similar to the MSD analysis used in the amphipod tests. Power analyses of the sea
urchin fertilization data have shown MSDs of 15.5% for alpha