Appendix A. 
B-IBI Sample Size and Standard Deviations on B-IBI 
Scoring when Screening Segments for Reference 
Community Characterization 
The EPA Chesapeake Bay Program Office in cooperation with its partners, examined the 
effects of relaxing the data screening criteria to accept segment-period combinations as 
“healthy” when defining reference communities with sample size > 8 (instead of the 
recommended n > 10) and/or standard deviation < 1.2 (instead of the recommended < 1.0). 
Data were 1996-2006 from the CIMS database. 
For the “fixed station” samples both “totalscore” and “grandscore” records were included. 
“Totalscore” records are replicate measurements of the same sampling event; the average of 
these is reported as the “grand score.” Benthic experts (Llanso, Versar, Inc.) recommend 
using the “grand score” in these analyses. Four scenarios were explored (Table A-l). The 
EPA accepted screening criteria is the default under Scenario A. Scenarios B, C and D 
relaxed the standard deviation, sample size and both sample size and standard deviation, 
respectively. 
Table Al. Healthy deep-water segments as characterized with four scenarios of screening 
criteria. The accepted screening criteria is Scenario A. __ 
Scenario A 
Scenario B 
Scenario C 
Scenario D 
(Default) 
B-IBI >3.0 
B-IBI >3.0 
B-IBI >3.0 
B-IBI >3.0 
n > 10 
n > 10 
n > 8 
n> 8 
S.D. < 1.0 
S.D. < 1.2 
S.D. < 1.0 
S.D. < 1.2 
Total number of 
“healthy” deep water 
10 
11 
13 
16 
segment-periods 
Relaxation of the criteria results in moderate increases (ranging from 1 to 6) in the number of 
segment-periods classified as “healthy.” Due to the increased risk of inaccurate 
classification, it is important to examine not just the number of additional segment-periods, 
but also the shape of these curves. If a curve is classified as “healthy” but its location in CFD 
space is consistent with DO violation CFDs of segment-periods classified as “degraded,” 
then it is reasonable to question whether an inaccurate classification has occurred. 
In the case of Scenario B (relaxing the standard deviation criterion from a maximum of 1.0 to 
a maximum of 1.2), a single curve (CB5MH 1999-2001) is added to the group of “healthy” 
segment-periods. In Figure A-l below, this curve is visible as a light blue line, while the 
population of 10 curves identified in Scenario A are presented by dark blue lines. Degraded 
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