At the end of the normalization step it is available a set of data composed by the normalized scores associated with each criterion. Two main approaches are possible to aggregate the scores:
- Simple sum
- Weighted sum.
In the case of the simple sum, the normalized scores are just summed to define the total score. The weight of a criterion depends on the number of points associated to it. For instance in a hypothetic assessment tool the criterion “Primary energy consumption” gives 10 points and the criterion “Potable water consumption” gives 5. In this case “Primary energy consumption” has the double weight with regards to “Potable water consumption”. The simple sum aggregation method is usually associated to a normalization procedure based on performance thresholds. In the case of the weighted sum aggregation method, a weight (usually expressed in percentage) is associated to each criterion. The total score is then calculated as the sum of the weighted scores of all criteria (weight time score).
The weighted sum aggregation method is usually associated to a normalization procedure based on linear performance scales.