Common use of Statistical Analyses Clause in Contracts

Statistical Analyses. A statistical mixture model was used for the analysis of the harborwide stormwater outfall data. A detailed description of the model (including the key assumptions, limitations, and statistical output) is provided in Appendix C. The model was used to analyze the distribution of the geometric mean concentrations for harborwide outfall basins and to assign each outfall basin to one of three concentration groups (i.e., lower, moderate, or higher). This concept is consistent with the JSCS method for identifying low, medium, and high priority sites. Statistical analysis was performed for the analytes selected based on the data screening described in Section 4.2 (unless the number of samples was insufficient to conduct a valid statistical analysis; see Table 4-1). Graphs of the results of the statistical evaluation are included in Appendix C. Use of the full harborwide data set in the statistical evaluation allowed comparison of stormwater concentrations in City outfall basins to other stormwater discharges in the Portland Harbor Study Area. Source tracing categories were statistically identified based on specific outfall concentrations relative to harborwide concentration levels. For 5 See data tables in Appendices A and B for data collected by the City; see Anchor and Integral 2008b for City outfall basin data collected by others. each analyte included in the statistical evaluation, each outfall basin was assigned to one of three source tracing categories: 1. Stormwater analyte concentrations in these basins are lower relative to stormwater concentrations harborwide (i.e., relative to other private and public outfalls) and are least likely to have significant uncontrolled sources. The need for additional source tracing in City outfall basins within this category is low.  Category 2. Stormwater analyte concentrations in these basins generally fall within a narrow moderate range and may or may not have significant uncontrolled sources. The need for additional source tracing in City outfall basins within this category should be evaluated further, unless otherwise indicated (see discussion below in Section 4.4).  Category 3. Stormwater analyte concentrations in these basins are higher relative to harborwide concentrations and are the most likely to have significant uncontrolled sources. The need for additional source tracing in City outfall basins in this category is evaluated. The break lines defining Categories 1 through 3 for each analyte are described and shown on the graphs in Appendix C. Table 4-2 summarizes the resulting distributions of source tracing categories by analyte and City outfall basin. Analytes identified as Category 2 or 3 source tracing categories were further evaluated to assess whether additional source tracing is needed in Section 4.4.