We consider the evaluation of a business process quality, in particular the evaluation of its robustness. By robustness, we mean robustness w.r.t. the risk of loosing knowledge of persons implied in the business process. We define metrics taking tacit knowledge into account. These metrics are based on the analysis of a social network underlying the process execution. We illustrate these metrics on a -real- application case: the evaluation of an IS project management business process.