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

3.2.9. Symptoms

Concept
Biological measurements (often at lower levels of biological organization than the effect) can be characteristic of one or a few specific causes.

Figure 3-11. Symptoms. The impairment (low fish production) may be caused by many agents, but only a few cause the specific symptom (spinal deformity). Observation of that symptom supports candidate causes that produce that symptom (e.g., selenium). If a known symptom is not observed, the case for the candidate cause is weakened.
(General explanation of symbols)

Examples
Consider increased levels of endocrine disrupting chemicals as a candidate cause of reduced fish abundance. What findings support or weaken the case for increased endocrine disruptors as the cause, based on symptoms?

• Supporting evidence - Necropsies of fish reveal the presence of ovotestes, and vitellogenin in serum from males. These findings are unique characteristics associated with exposure to endocrine disrupting chemicals.
• Weakening evidence - Necropsies of fish do not reveal the presence of ovotestes or vitellogenin; rather, they show that the fishes' stomachs have been tinted blue, a characteristic symptom of molybdenum toxicity.

How do I analyze the data?
The presence or absence of characteristics that occur only in response to a particular stressor can be used to diagnose that stressor as the cause. Confidence in this type of evidence is increased when a larger number of characteristic symptoms are observed, or when the observed symptoms are highly specific to few potential causes. Non-specific effects are more difficult to diagnose, so this type of evidence is more helpful when impairments are defined as specifically as possible (e.g., as decreases in specific insect taxa of concern, rather than as decreases in total insect abundance).

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

• Data showing that a unique set of characteristics caused by a candidate cause (e.g., symptoms within the organism, the presence of indicator species) are present at the impaired site

Weakens

• Data showing that one or more characteristics usually caused by a candidate cause are not present at the impaired site
• Data showing that one or more characteristics at the impaired site that are not those caused by the candidate cause are present at the impaired site

How do I score the evidence?

Helpful tips

• Diagnostic symptoms are reasonably well documented for fish kills, but currently are not as well-developed for invertebrates.
• Although the term "symptoms" is familiar from its use in medicine, the concept can be extended to other levels of biological organization. For example, when effects are defined at the assemblage level (e.g., decreased numbers of mayfly taxa) the abundances of specific taxa can be analyzed as symptoms in support of particular candidate causes.