Guideline 12: Discriminatory Ability 



The ability of the indicator to discriminate differences among sites along a known condition gradient 

 should be critically examined. This analysis should incorporate all error components relevant to the 

 program objectives, and separate extraneous variability to reveal the true environmental signal in the 

 indicator data. 



Performance Objective 



1 . Demonstrate responsiveness of the indicator and its component metrics to individual 

 stressors or to the cumulative effects of multiple stressors. 



Conceptual relationships between the indicator and its component metrics and various types of stressors 

 have been addressed (Guideline 2). Other studies using similar multimetric indicators have demonstrated 

 the potential responsiveness of the indicator (Table 4-17). For this indicator, a large number of sites, 

 representing a range of stressor intensities, are used rather than an experimental-based approach using 

 sites of known stress intensity. The proposed evaluation approach for this guideline is graphic (Fig. 4-8), 

 rather than statistical (Fore et al. 1 996, Karr and Chu 1 997). Indicator values or individual metric scores are 

 plotted against individual stressor variables, and/or against new variables derived from multivariate analyses 

 of suites of stressor variables (e.g., Hughes et al. 1998). 



Table 4-17. Responsiveness of other multimetric fish assemblage indicators to stressors 



• Karr et al. (1985): Chlorine 



• Steedman (1988): Gradient of urban to forest land use 



• Rankin (1995): Habitat quality in Ohio (Correlation coefficients between 0.45 and 0.7) 



• Wang et al. (1997): Land use in Wisconsin 



• Hughes et al. (1998): Intensity of human disturbance 



Summary 



Individual metrics respond predictably to specific stressors, though in some cases those specific responses 

 are weak. The individual metrics and the indicator exhibit the predicted responsiveness to a multivariate 

 "disturbance" variable derived from several individual chemical, habitat, and watershed stressor variables. 

 Individual metric responses to specific stressor variables, as well as expectations for scoring metrics, should 

 be examined to determine if responsiveness of the indicator to suites of stressor variables can be improved. 



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