Scoring the Evidence: Background
After you have assembled and analyzed the data, we recommend that you evaluate and score the evidence to summarize the degree to which it supports or weakens the case for the candidate cause. The scoring system is available in summary form (summary table of scores). Scores are also presented on the information page for each type of evidence (e.g., spatial/temporal co-occurrence). The rationale for each score is provided in the column entitled "interpretation". The sign of the score is based on whether the type of evidence supports the candidate (+), weakens the candidate (-) or has no impact (0).
The number of plusses and minuses increases with the degree to which the evidence either supports or weakens the case for a candidate cause. Evidence can score up to three plusses (+++) or three minuses (---). However, the maximum number recommended for a particular type of evidence depends on the likelihood that an association might be observed because of chance rather than because of the true cause. Therefore, the highest scores are given to the types of evidence:
- That use data from the site,
- That are based on more than one association,
- That closely link the proximate cause and the effect.
If the available data cannot be analyzed in way that can be used to evaluate a type of evidence, it is scored as "no evidence" (NE). If other candidate causes do have this type of evidence, we recommend including the NE to help compare the relative strength of the evidence across candidate causes. However, if no candidate causes can be evaluated for a particular type evidence, we recommend that you discuss the lack when evaluating overall confidence, but do not include the row of NEs in your summary scoring table. Sometimes it doesn't make sense to score one type of evidence, because of the results of another. For example, it wouldn't make sense to evaluate a stressor-response relationship in the field if the effect and stressor do not spatially co-occur. In these situations, we recommend using "not applicable" (NA).
There are two other types of scores:
- Refute (R) is used for indisputable evidence that disproves that the candidate cause is responsible for the specific effects.
- Diagnose (D) is used when a set of symptoms for a particular causal agent or class of agents is, by definition, sufficient evidence of causation, even without the support of other types of evidence.
Once you have all the scores, compile them in a summary worksheet (example scoring table). These scores are used in Step 5 to identify the probable cause.