can be determined to be significant. These confidence intervals are a function of both the variability in the 

 data and the sampling design. Alternative approaches might be needed to evaluate the utility of this indicator 

 for programs with significantly different designs. 



Comparisons between curves can be made for those generated in two different regions {i.e., status 

 comparison) or from the same region at two different times {i.e., trends comparison). Although this analysis 

 does not separate out variability due to extraneous factors, it does provide insight into the utility of the 

 indicator to discriminate condition using the design of the EMAP-Virginian Province program. 



Phase 4: Interpretation and Utility 



Once it is determined that the indicator is relevant, applicable, and responsive, the final phase of evaluation 

 is to determine if the results can be clearly understood and useful. 



Guideline 13: Data Quality Objectives 



The discriminatory ability of the indicator should be evaluated against program data quality objectives 

 and constraints. It should be demonstrated how sample size, monitoring duration, and other variables 

 affect the precision and confidence levels of reported results, and how these variables maybe optimized 

 to attain stated program goals. For example, a program may require that an indicator be able to detect 

 a twenty percent change in some aspect of ecological condition over a ten-year period, with ninety-five 

 percent confidence. With magnitude, duration, and confidence level constrained, sample size and 

 extraneous variability must be optimized in order to meet the program 's data quality objectives. Statistical 

 power curves are recommended to explore the effects of different optimization strategies on indicator 

 performance. 



The Data Quality Objective for trends in EMAP-Estuaries was to be able to detect a two percent change per 

 year over 12 years with 90% confidence. This indicator meets that requirement as shown in Figure 2-1 1 . 

 This figure shows several power curves for annual changes ranging from one to three percent. Note that 

 these curves are based on data from more than 400 stations sampled over a period of four years. The ability 

 to detect trends will differ using different sampling designs. If fewer stations were to be sampled, a new set 

 of power curves could be generated to show the ability to detect trends with that number of stations. 



Guideline 14: Assessment Thresholds 



To facilitate interpretation of indicator results by the user community, threshold values or ranges of 

 values should be proposed that delineate acceptable from unacceptable ecological condition. Justification 

 can be based on documented thresholds, regulatory criteria, historical records, experimental studies, 

 or observed responses at reference sites along a condition gradient. Thresholds may also include 

 safety margins or risk considerations. Regardless, the basis for threshold selection must be documented. 



Although there is debate regarding their validity, assessment thresholds already exist for dissolved oxygen. 

 Several states have adopted 5 mg/L as a criterion for 24-hour continuous concentrations and 2 mg/L as a 

 point-in-time minimum concentration for supporting a healthy ecosystem. This is supported by EPA research 

 (U.S. EPA 1998) which shows long-term effects at 4.6 mg/L and acute effects at 2.1 mg/L. If these thresholds 

 change in the future, data collected on this indicator can easily be re-analyzed to produce new assessments 

 of the hypoxic area. 



2-15 



