Figure VI-9. Light area reflects amount of 'allowable' criteria exceedance defined as 
the area under the reference curve (light line). Dark area reflects the amount of 'non¬ 
allowable' criteria exceedance defined as the area between the attainment 
curve (black line) and the reference curve. 
In cases where the amount of ‘non-allowable exceedances’ is large (e.g., Figure VI- 
8, line c; Figure VI-9), decisions regarding the attainment of designated uses will be 
unequivocal. However, situations could arise where small amounts of non-allowable 
exceedance could render the decisions less clear. Figure VI-10 illustrates a situation 
in which a decision on nonattainment might be clear (a) and one in which the deci¬ 
sion might be less clear (b). In the latter case, questions could arise about the 
certainty of the analysis and whether the data were adequate to unequivocally decide 
that the portion of the Chesapeake Bay was not attaining its designated use. In some 
cases, many data points could have contributed to the development of the CFD, 
whereas in other cases there may have been only a few. It is possible to define the 
decision rule that any non-allowable exceedance would indicate nonattainment of 
the established designated use. However, a decision rule based on a statistical test 
could help to address some of the uncertainty involved by accounting for differences 
in the number of observations on which the analysis is based. 
Work is currently under way to devise a statistical test for the application of CFDs 
to assess water quality criteria attainment in the Chesapeake Bay. The test currently 
chapter vi • Recommended Implementation Procedures 
