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biostatisticians and environmental and regulatory scientists. Scientific interest will 
also be garnered by public and stakeholder interest. The CFD approach here presents 
a challenge as it is complex in explanation. Still with careful diagrams and examples, 
a brochure on the CFD approach should be extremely useful in getting uninitiated 
scientists and stakeholders on the same page. 
6.3. BIOLOGICAL REFERENCE CURVES 
The success of the CFD-based assessment will be dependent 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. The reference and attainment curves 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 curve allows for implemen¬ 
tation of threshold uncertainty as long as the reference curve is sampled similarly to 
the attainment curve. Bias and uncertainty are driven in CFD curves by sample 
densities in time and space. Therefore, we advise that similar sample densities are 
used in the derivation of attainment and reference curves. As this is not always 
feasible, analytical methods are needed in the future to equally weight sampling 
densities between attainment and reference curves. 
Conceptually, the CFD approach builds on the underlying view that water quality 
criteria are surrogates for Designated Uses (regions that define ecosystem function). 
Implicit is a bottom up model based upon eutrophication, which is expected to 
diminish the designated use. Reference curves represent thresholds related to the 
functioning of designated use regions. Therefore, choice of reference regions or 
periods and sampling design in developing reference curve is critical to the imple¬ 
mentation of a scientifically-rigorous CFD approach. Choice of such regions is 
beyond the scope of this review, but we emphasize several relevant statistical issues 
in developing reference curves in Section 4. 
7.0 RECOMMENDATIONS FOR FUTURE EVALUATION 
AND REFINEMENT OF THE 
CFD ASSESSMENT METHODOLOGY 
As part of its conclusions, the STAC CFD Review Panel identified critical remaining 
issues that need resolution in the near future. The following is a list of critical aspects 
of that needed research. These research tasks appear roughly in order of priority. 
However, it must be recognized that it is difficult to formulate as set of tasks that can 
proceed with complete independence. For example, research on task 1 may show 
that the ability to conditionally simulate the water quality surface is critical to 
resolving the sample size bias issue. This discovery might eliminate IDW as a choice 
of interpolation under task 3. The Panel has made significant progress on several of 
these research tasks and CBP is encouraged to implement continued study in a way 
that maintains the momentum established by this research group (Table 7.1.). 
appendix a 
The Cumulative Frequency Diagram Method for Determining Water Quality Attainment 
