11 
Step 1. Collect data at 
known locations. 
Date 
3 
3 
5 
2 
1 
Date 2 
1 
1 
3 
1 
1 
Date 3 
4 
2 
2 
1 
1 
Date 4 
1 
4 
2 
4 
1 
Date 5 
1 
3 
2 
1 
1 
Step 2. Interpolate the 
data to grid cells. 
Date 
3 
4 
5 
3 
4 
4 
5 
2 
3 
3 
4 
1 
2 
3 
3 
1 
Date 2 
1 
2 
3 
1 
2 
2 
3 
2 
1 
3 
2 
1 
1 
1 
1 
1 
Date 3 
4 
3 
2 
2 
3 
2 
2 
1 
2 
2 
1 
1 
1 
1 
1 
1 
Date 4 
1 
2 
3 
4 
2 
2 
2 
3 
3 
3 
2 
1 
4 
3 
1 
1 
Date 5 
1 
2 
3 
3 
2 
2 
2 
2 
1 
1 
1 
1 
1 
1 
1 
1 
Steps 3-4. Determine 
attainment status of 
each cell. 
Date 
1 
1 
1 
1 
1 
1 
1 
0 
1 
1 
1 
0 
0 
1 
1 
0 
Date 2 
0 
0 
1 
0 
0 
0 
1 
0 
0 
1 
0 
0 
0 
0 
0 
0 
Date 3 
1 
1 
0 
0 
1 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
Date 4 
0 
0 
1 
1 
0 
0 
0 
1 
1 
1 
0 
0 
1 
1 
0 
0 
Date 5 
0 
0 
1 
1 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
Step 5: Determine 
percent attainment by date. 
Sample date 
Percent 
space 
Date 1 
75.00% 
Date 2 
18.75% 
Date 3 
18.75% 
Date 4 
43.75% 
Date 5 
12.50% 
Step 6. Rank the percent of 
space values and assign percent 
of time as (100*R/(N+1)), where 
R is rank and N is sample size. 
Sample date 
Ranked 
percent 
space 
Percent time 
100% 
0% 
Date 1 
75.00% 
16.67% 
Date 4 
43.75% 
33.33% 
Date 2 
18.75% 
50.00% 
Date 3 
18.75% 
66.67% 
Date 5 
12.50% 
83.33% 
0% 
100% 
Steps 7 and 8. Figure II-3 illustrates the plot of this 
theoretical assessment CFD and the comparison to a 
hypothetical reference curve. In this hypothetical ex¬ 
ample, the assessment area shows non-attainment. For a 
percent area of 18.75, the allowable frequency on the 
reference curve is about 17 percent. That is, 18.75 per¬ 
cent of the segment area should not be out of attainment 
more that 17 percent of the time. For Date 3, the esti¬ 
mated frequency of 18.75 percent of segment area in 
non-attainment is 66.67 percent. Thus the frequency of 
18.75 percent of space out of attainment exceeds the 17 
percent allowed. The reference curve is exceeded for 
dates 4 and 1 as well. 2 
2 ln this cumulative distribution framework, the actual date is 
not relevant. One should not infer that non-attainment occurred 
on that date if the data point associated with a date falls above 
the reference. The date is used here as a label for each coordi¬ 
nate pair. 
chapter ii 
Refinements to the Chesapeake Bay Water Quality Criteria Assessment Methodology 
