Calculating Individual Threat Ratings Based on Scope, Severity and Irreversibility
Miradi combines scope and severity ratings to get the overall threat magnitude rating for each threat on each target, using the following rule-based system:
Miradi then combines the threat magnitude rating with the irreversibility rating using the following rule-based system:
Rolling Up Ratings
Miradi uses a rule-based procedure for aggregating threat ratings across multiple targets or across multiple threats. Miradi’s rules for rolling up threat ratings were developed by the TNC 5-S Framework and have been applied in threat ratings carried out by hundreds of TNC teams across the globe.
Miradi creates a matrix of threats and targets, as shown in Figure 1. In this example, the far right-hand column contains the rankings for each threat across targets (a Type II roll-up). The bottom row contains the overall threat ranking for each target (a Type III roll-up). To calculateType II and Type III roll-ups, Miradi uses two rules:
The 3-5-7 rule:
- 3 High ranked threats are equivalent to 1 Very High-ranked threat
- 5 Medium ranked threats are equivalent to 1 High-ranked threat
- 7 Low ranked threats are equivalent to 1 Medium-ranked threat
The 2-prime rule: This rule requires the equivalent of two Very High rankings (e.g., one Very High and at least three High rankings) for the overall ranking to be Very High and the equivalent of two High rankings for the overall ranking to be High.
Figure 1 shows examples of the application of these rules. In the second row, the Housing threat has 3 High rankings (which equals 1 Very High) and 1 Very High ranking. Thus, the overall Threat Rank is Very High. Likewise, in the Upper Watershed Column, there are 6 High rankings, which equal 2 Very High rankings. Thus, the overall rank for this target is Very High. And finally, the cell in the lower right-hand corner contains the overall ranking for the project (a Type IV roll-up), which is calculated by rolling up the far-right hand column using the 2- prime rule.