Manipulability, Unanimity, Anonymity and Assesment Aggregation.

An assesment aggregation rule is a mechanism which collapses several individual assessments ("quantitative opinions") into a unique ("collective") assessment. Individual i can manipulate assessment aggregation if the change in the aggregated assessmentdue to a change in i's assesment from x to y. Some asumptions on the functionl form capturing this dependance are put forward and their effects on aggregationinvestgated under unanimity and anonymity conditions. Threee results are sought after: (1)the aggregation rule does not exist; (2) it is the arith,etical mean; (3) it is dictatorial.