Causal Analysis/Diagnosis Decision Information System (CADDIS)

Step 3: Evaluate Data from the Case

• 3.1. Overview
• 3.2. In-Depth Look
• 3.3. Results and Next Steps

3.2.3. Causal Pathway

Concept
Steps in the pathways linking sources to the cause can serve as supplementary or surrogate indicators that the cause and the biological effect are likely to have co-occurred.

Figure 3-5a. Causal Pathway, Supports. The impairment (dead fish) occurs downstream of the source of the precursor (phosphate) and high algal production is sufficient to cause high early morning respiration resulting in observed low dissolved oxygen (the causal agent). Turbulence, which might have aerated the water, is absent.
(General explanation of symbols)

Figure 3-5b. Causal Pathway, Refutes. The impairment (dead fish) occurs downstream of the source of a precursor (phosphate) of the candidate causal agent (dissolved oxygen) but the proposed causal pathway is interrupted by turbulence which aerates the water, and the lack of periphyton on the cobble substrate confirms that there is no eutrophication, so another step is absent.

Examples
Consider low concentrations of dissolved oxygen as a candidate cause of decreased fish abundance. One of several causal pathways by which dissolved oxygen concentrations can be reduced is via an increase in nutrients allowing increased accumulation of algal biomass. When these algae eventually die, bacteria, fungi and protozoans can increase and rapidly consume the available oxygen. Given this causal pathway, what findings support or weaken the case for low levels of dissolved oxygen as the cause?

• Supporting evidence - Monitoring data show that sites with low fish abundance have higher nutrient concentrations or greater algal biomass than sites with high fish abundance.
• Weakening evidence - Monitoring data show that nutrient concentrations and algal growth measures are not higher at sites with reduced fish abundance, relative to unimpaired sites.

How do I analyze the data?
Data relevant to the hypothesized steps linking a candidate cause to potential sources can be used to assess the likelihood that that agent is present. These steps in the causal pathway serve as surrogates for the proximate stressor when data on the stressor itself are unavailable or as supplementary sources of information when stressor data is available. Multiple causal pathways may lead to a candidate cause, and evidence supporting the steps in even one pathway can be enough to bolster the case for a candidate cause.

Candidate causes cannot be refuted using causal pathway evidence. Although some pathways may be eliminated, there are always potential unknown sources or pathways that may result in the candidate cause.

The analytical challenges inherent in evaluating spatial/temporal co-occurrence also apply here.

• Example analysis worksheet for causal pathway:
  • Little Scioto River, OH, USA

What evidence would support or weaken the case for a candidate cause?
Supports

• Data showing that one or more steps in the causal pathways linking sources to the candidate cause are present at the impaired site, relative to unimpaired sites

Weakens

• Data showing that one or more steps in the causal pathways linking sources to the candidate cause are not present at the impaired site, relative to unimpaired sites

How do I score the evidence?

Helpful tips

• Save data directly related to the proximate stressor for analysis under spatial/temporal co-occurrence or evidence of exposure or biological mechanism, as these types of evidence carry more weight.
• Causal pathway evidence considers the intermediate steps between sources and the proximate stressor, whereas evidence of exposure or biological mechanism considers the intermediate steps between the proximate stressor and the observed biological effect.
• Keep in mind that causal pathway evidence cannot refute the case for a candidate cause, because although critical steps in some pathways may appear to be absent, there may be unknown pathways or unknown system dynamics operating.