data) while benthic community assessments are conducted annually with annual time steps for a 
variety of purposes (e.g. indicator reporting for the Chesapeake Bay Barometer). Using 
sequential 3-year time periods to classify benthic community health, advancing the data in one 
year time steps (e.g. 1996-1998, 1997-1999, etc.), brings the reference community identification 
method into better alignment with the dissolved oxygen criteria assessment protocols for which 
reference communities are being identified. This modification addresses a concern raised by the 
Chesapeake Bay Program's Scientific and Technical Advisory Committee (STAC) review of the 
CFD approach (STAC 2006) which noted that sample sizes for reference and assessed conditions 
should be made similar to reduce the effect of sample size bias on the shape of the CFD. The 
combination of a segment and sequential 3-year assessment time periods is hereafter referred to 
as a “segment period”. 
Screening criteria: Sample Size > 10 
Keller and Cavallaro (2008) reported that listing decisions on the U.S. Clean Water Act 303a 
listing impairments of surface waters by states were often based on insufficient data, or that data 
were not sufficiently representative of temporal and spatial conditions for the water body being 
assessed. Llanso et al. (2009), however, require a minimum sample size of n > 10 for habitat 
health assessments using the Chesapeake Bay B-IB1. The EPA Chesapeake Bay Program Office 
and its partners examined the effects of relaxing the data screening criteria to accept segment- 
period combinations with sample size > 8 to increase the number of “healthy” segment-periods 
available for reference community analysis. 
The decision to eliminate segment-periods with fewer than 10 observations was based on 
analyses by EPA Chesapeake Bay Program Office and its partners, which showed that fewer 
than 10 observations weakened the ability of the reference CFD to appropriately classify 
segments. Llanso et al (2009) confirmed Keller and Cavallaro (2008)’s findings regarding 
sample size and temporal and spatial distribution. They found that analysis of Chesapeake Bay 
segments with less than 10 samples produced “inconclusive results relative to the (U.S. EPA) 
listing process.” In their review of the proposed methodology, STAC (2009) determined that a 
minimum sample size of 10 is reasonable and has been applied elsewhere (Alden et al. 2002). 
Further details on the sample size analyses are available in Appendix A. 
Screening Criteria: Standard Deviation <1.0 
The EPA Chesapeake Bay Program Office and its partners examined the isolated and combined 
effects of relaxing the data screening criteria to accept segment-periods with fewer samples (n > 
8 instead of 10) and/or expanding the standard deviation criteria surrounding the B-IBI results 
from <1.0 to < 1.2 in order to increase the number of “healthy” segment-periods for analysis. 
The relaxation of both the sample size and the standard deviation criteria (see Scenario D in 
Appendix A) increases the number of segment-periods classified as “healthy” from 10 to 16. 
Flowever, four of these additional CFD curves extend into “degraded” CFD space to a degree 
that calls into question the accuracy of their classification as healthy (see Figure A-3 in 
Appendix A). Defining healthy benthic communities for deriving a benthic community based 
biological reference curve, therefore, relies on sample size n > 10 with a standard deviation < 
19 
