• loss of information. For example, aggregation suppresses the fact that alternative A has environmental effects, whereas alternative B has financial effects;
• subjective weighting of different aspects. Unfortunately, these crucial weights and assumptions are often implicit or highly speculative. They may impose to the decision makers a value scheme bearing little relation to their concerns. For example, cost-benefit analysis implicitly assumes that a dollar's worth of one kind of benefit has the same value as a dollar's worth of another. Yet in many public decisions, monetary equivalent but otherwise dissimilar benefits would be valued differently by society.
• impossibility to aggregate for several clients with different points of view. The aggregate techniques are intended to help an individual decision maker in selecting the preferred alternative, the one that best reflects his values (importance weights). Serious theoretical and practical problems arise when there are multiple decision makers: Whose values should be used (the issue of inter-personal comparison of values), and what relative weight does the group give to the preferences of different individuals (the issue of equity)?
• assumption of independence effects. The aggregate technique (others than cost-benefit analysis) requires that the importance (value) of each effect be independent of the size of all other effects. But in the real world, this condition is not always satisfied. Each effect that violates this condition must be suppressed, either by eliminating it or by treating is at the next level of aggregation.