A-2 
EXECUTIVE SUMMARY 
BACKGROUND AND ISSUES 
In accordance with the Chesapeake 2000 Agreement, the Chesapeake Bay Program 
has recently implemented important modifications to (1) ambient water quality 
criteria for living resources and, (2) the procedures to determine attainment of those 
criteria. A novel statistical tool for attainment, termed the Cumulative Frequency 
Diagram (CFD) approach, was developed as a substantial revision of previous attain¬ 
ment procedures, which relied upon a simple statistical summary of observed 
samples. The approach was viewed as advantageous in its capacity to represent 
degrees of attainment in both time and space. In particular, it was recognized that the 
CFD could represent spatial data in a synoptic way: data that is extensively collected 
across diverse platforms by the Chesapeake Bay Program Water Quality Monitoring 
Program. Because the CFD approach is new to Bay Program applications, under¬ 
lying statistical properties need to be fully established. Such properties are critical if 
the CFD approach is to be used to rigorously define regional attainments in the 
Chesapeake Bay. 
In Fall 2005, the Chesapeake Bay Program Scientific, Technical and Advisory 
Committee charged our working group to provide review and recommendations on 
the CFD attainment approach. As terms of reference we used guidelines of Best 
Available Science recently published by the American Fisheries Society and the 
Estuarine Research Federation. Statistical issues that we reviewed included, 
1. What are the specific analytical/statistical steps entailed in constructing CFD 
attainment curves and how are CFDs currently implemented? (Section 2) 
2. How rigorous is the spatial interpolation process that feeds into the CFD 
approach? Would alternative spatial modeling procedures (e.g., kriging) 
substantially improve estimation of water quality attainment? (Section 3) 
3. What are the specific analytical/statistical steps entailed in constructing CFD 
reference curves? (Section 4) 
4. What are the statistical properties of CFD curves? How does sampling density, 
levels of attainment, and spatial covariance affect the shape of CFD curves? 
What procedures are reliable for estimating error bounds for CFD curves? 
(Section 5) 
5. From a statistical viewpoint, does the CFD approach qualify as best available 
science? (Section 6) 
6. What are the most important remaining issues and what course of directed 
research will lead to a more statistically rigorous CFD approach over the next 
three years? (Section 7) 
The central element of our work was a series of exercises on simulated datasets 
undertaken by Dr. Perry to better evaluate 1) sample densities in time and space, 2) 
varying levels of attainment, and 3) varying degrees of spatial and temporal 
appendix a 
The Cumulative Frequency Diagram Method for Determining Water Quality Attainment 
