A weighted index or a value model is scale that weights and combines different impacts. For example, if there are different types of affected wildlife habitat, calculate a weighted summary score or index of the overall effect by: 

• Estimating the impact on each criterion (each habitat type) under each alternative. 

• Assigning a weight (or importance) to the impact (W). 

• Calculating a score for each alternative that is the weighted sum across habitat types (a, b, c):
   Score for Alternative 1 = Wa x Aa + Wb x Ab + Wc x Ac

In general, the steps for developing a weighted index are: 

• Estimate the impact on each criterion for each alternative. This is a technical judgment. 
• Assign weights to each criterion. Assign them only once. This is a mix of value and technical judgments. 
• Use the weighted score (1000) to compare alternatives. It can be interpreted as weighted hectares, or in this case, equivalent hectares of Habitat a. 
• If the attributes are reported in different units, then you must normalize criterion scores before weighting.

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