translate English to French: It is difficult for various reasons to carry out such appraisals: on the one hand they depend on clear assessment criteria, and on the other they require indicators which are precisely quantifiable.
Pour différentes raisons, il s'agit d'un exercice difficile supposant la fixation préalable de critères clairs et précis d'une part et, d'autre part, d'indices parfaitement mesurables.