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In general, the STAC panel analysis and review indicated that the CFD approach can 
combine spatial and temporal data to support inferences on attainment or exceedance 
of the water quality criteria (STAC 2006). The panel viewed the CFD approach as 
innovative — one that has general application in water quality criteria assessments. 
In comparison to other jurisdictional authorities, the Chesapeake Bay Program has 
taken a lead in monitoring and assessment based upon scientific design (designated 
uses) and emphasis on statistical evidence. Advancement in the CFD approach 
should provide an important precedent for states outside the Chesapeake Bay region. 
Because the CFD is both feasible and innovative, the panel felt that it qualifies as the 
best available approach. On the other hand, the panel recognized that the approach 
remains nascent and deserves further directed study and analyses to evaluate the bias 
and imprecision that can occur due to small sample densities, non-independence in 
temporal trends, and inadequate spatial interpolations. 
The panel found that the CFD approach in its current form is feasible, but requires 
additional research to further refine and strengthen it as a statistical tool. The CFD 
builds on important statistical theory related to cumulative distribution functions; as 
such, its statistical properties can be simulated and deduced. In its analyses, the 
STAC panel showed that constructing confidence ellipses that support inferences 
related to threshold curves or other tests of spatial and temporal compliance are 
feasible. Understanding fundamental properties of how the CFD represents likely 
covariances of attainment in time and space and how temporal and spatial correla¬ 
tions interact with sample size effects require additional research. Further, 
researchers must also analyze biases across regions and designated-use segments. 
The panel expects that two to three years of directed research and development are 
necessary to identify and measure potential sources of bias and imprecision for 
criteria attainment determinations. 
In the near future, the panel foresees that the CFD approach will prove particularly 
powerful when linked to continuous spatial data streams through the cruise-track 
monitoring program, and when able to utilize continuous temporal data generated 
through further deployment of remote sensing platforms in the Chesapeake Bay 
(e.g., Chesapeake Bay Observing System). These data sets will allow greater preci¬ 
sion and accuracy in both threshold and attainment determinations made using the 
CFD approach. 
The STAC panel concluded that success of the CFD-based assessment rests upon 
decision rules related to the biological reference curves. These curves represent 
desired segment-designated use water quality outcomes and reflect sources of 
acceptable natural variability (STAC 2006). The reference and attainment curves 
should follow the same general approach in derivation: water quality data collection, 
spatial interpolation, comparison to biologically based water quality criteria, and 
combination of space-time attainment data through a CFD. Therefore, the biological 
reference curves allow implementation of a tolerance threshold presuming the data 
used to derive the reference curve were sampled similarly to the assessment curve. 
chapter ii 
Refinements to the Chesapeake Bay Water Quality Criteria Assessment Methodology 
