CADDIS Volume 1: Stressor Identification
Step 3: Evaluate Data from the Case
In Step 3, you assemble and analyze data from the case at hand, with two goals in mind:
- To develop consistent and credible evidence that allows you to confidently eliminate very improbable causes, or to use symptoms to refute or diagnose a cause, and
- To begin building the body of evidence for those candidate causes that cannot be eliminated or diagnosed, which will be used in Step 5 to identify the most probable causes.
Analyses conducted during this step combine measures of the biological response (e.g., trout abundance or invertebrate taxonomic richness) with direct measures of proximate stressors (e.g., toxicant concentrations or percent embeddedness values), or measures of other steps linking sources, candidate causes, and biological effects. For example, if low levels of dissolved oxygen (DO) constitute the candidate cause, data from the case may include actual dissolved oxygen measurements at the impaired and reference sites; evidence that organisms intolerant of low DO have declined at the impaired site; and/or measurements of increased organic matter (one potential step in the causal pathway) at the impaired site.
- Do the candidate cause and the effect occur in the same location?
- Is there a complete series of events linking the source to the causal agent?
- Does the magnitude of the effect increase with the magnitude of exposure to the causal agent?
Questions that less frequently can be addressed using data from the case
- Did exposure to the candidate cause precede the effect?
- Are there biomarkers or other mechanistic indicators linking exposure to the proximate stressor to the adverse biological effect?
- Does the biological effect change in response to intentional manipulation of the stressor in the case?
- Are there diagnostic symptoms or patterns?
Each type of evidence should be evaluated and you should document the analysis and results in a series of worksheets. The degree to which each type of evidence supports or weakens a case is scored using a standard system. Data from the case may show that it is impossible or extremely improbable that a candidate cause produced the observed effect; if this happens, that candidate cause can be eliminated from further consideration. Certain symptoms may allow for a confident diagnosis or refutation of a candidate cause. Scores for the candidate cause or causes that remain are carried forward, and combined with scores assigned after bringing in data from outside of the case in Step 4: Evaluate Data from Elsewhere.